The Chiltern Bubbles 2003-2018 and beyond

Back on April 7th 2003 the "Chiltern bubble" namely a heavily refurbished class 121 single car W55020 entered service with Chiltern Railways working peak hour Aylesbury - Princes Risborough shuttles. The use of an ex departmental unit freed up a 2 car "Turbo" for higher capacity peak hour requirements elsewhere on Chiltern network.

In May 2011, a second ex Departmental class 121 single car in W55034 joined the Chiltern fleet entering service on May 25th 2011 again on peak hour services between Aylesbury and Princes Risborough.

Sadly 14 consecutive years of Chiltern "bubble" operating ended in May 2017 bringing to an end of slam door DMU operation on Chiltern and throughout the national network. Still at 57 years old let's hope the Chiltern bubbles survive as route learners or in preservation for many more years to come

Early days

55020 was the first of an improved single car unit batch of 16 class 121 units built by Pressed Steel in Linwood, near Glasgow entering service for the Western Region of British Rail (being Southall/Reading based) in 1960. 55034 the penultimate class member was delivered a few months later again initially being a Western Region unit.

In lined green and later all over BR blue both 55020 and 55034 would have worked branch line services all over the Western Region. 55020 would have been a regular to what was to become Chiltern railways in later years working High Wycombe- Bourne End branch services right up to closure in May 1970. 55020 was to receive Chocolate and cream livery as part of the GWR 150 celebrations in 1985 bringing a touch of nostalgia to the Greenford branch and other duties the unit still worked. NSE stripes were applied during a Doncaster overhaul in October 1987 bringing a very different livery to the Greenford and Bicester Town branches plus main line duties in multiple with other DMU classes in the Paddington area for a few more years.

The Turbo invasion of Thames train services saw W55020 become a departmental "Sandite" unit by 1993. And then came the call from Chiltern.

W55034 was transfered to Tyseley in 1986 with this unit working classic Midland/Western turns like the Stourbridge Jn - Stourbridge Town and a more demanding all day diagram linking Worcester with Barnt Green via the Lickey. More modern stock displaced 55034 to departmental duties in 1992 until the call from Chiltern came in 2011.

55020's and 55034's rebuilding

The modifications undertaken at Tyseley included:

On 55020 removal the chunky exhaust pipes were removed from the front of the unit and relocated to under the guards compartment. 55034 didn't receive this major modification.

secondary door locking with door open indicators fitted

extra window in guards compartment

55020 received traditional blue livery with silver lining and Chiltern branding. 55034 received traditional green livery with 1960's British Railways logos.

New seating, improved heaters and a thorough mechanical overhaul of the 6 cylinder AEC 150 hp engines.

Chiltern bubble diagrams

Both 55020 and 55034 have normally worked morning and evening peak services on the 7 mile Aylesbury - Princes Risborough branch. Specific turns have varied but typical diagrams are shown below.

Applies Mondays to Fridays Only, Until May 19th 2017

2P09 0635 Aylesbury - Princes Risborough

2A12 0720 Princes Risborough - Aylesbury

2P16� 0747 Aylesbury - Princes Risborough

2A12 �0823 Princes Risborough - Aylesbury

2P23 0849 Aylesbury - Princes Risborough

2A15 0916 Princes Risborough - Aylesbury

2P23 0849 Aylesbury - Princes Risborough

2A15 0916 Princes Risborough - Aylesbury

5P46 1639 Aylesbury - Princes Risborough ECS

2A46 1700 Princes Risborough - Aylesbury

2P65 1726 Aylesbury - Princes Risborough

2A50 1806 Princes Risborough - Aylesbury

2P70 1828 Aylesbury - Princes Risborough

2A55 1930 Princes Risborough - Aylesbury


These diagrams have see a total mileage daily of 100 miles.


Speeding "Bubble"

The Aylesbury - Princes Risborough does not see the "bubbles" normally get anywhere near their 70 mph maximum speed. A 10mph restriction at Marsh Crossing midway between Aylesbury and Little Kimble brings down the average speed to around 28 mph for this longer section. Even non stop runs rarely see any fast running!


Table 1 55020 0849 Aylesbury - Princes Risborough January 20th 2017


Sched arr/dep

Actual arr

Actual dep








0849 dep




Little Kimble





Monks Risborough





Princes Risborough

0906 arr










The "Bubbles" on Tour


Whilst diagrammed only to work between Aylesbury and Princes Risborough the "bubbles" have on occasions strayed furthur afield. The regular "Quantonian" shuttles on early May and late August bank holidays linking Aylesbury with the Quainton Railway centre have seen either 55020 or 55034 shuttling along the largely freight only tracks between Aylesbury and Quainton well filled with happy punters. May 1st 2017 was the last time a bubble features on these shuttles

Railtours between Aylesbury and the preserved Chinnor branch via Princes Risborough saw rare mileage for 55020 on October 12th 2013 and 55034 a week later.

The Severn Valley Railways "Diesel railcar reunion" saw 55020 a star visitor on October 8th/9th 2010. 55020 worked ECS to and from Kidderminster making an unusual sight passing Birmingham Moor St on October 7th 2010.


Both 55020 and 55034 have been used as route learners in the Midlands with 55034 working Tyseley - Nuneaton via Bescot and Sutton Park route learner on Januar 24th 2014 for cross country trains staff. 55020 worked a Tyseley - Crewe route learner on February 5th 2013.

More recently 55034 in multiple with a 117 DMU and fellow ex Arriva trains class member 55032 worked Aylesbury - Tyseley via Princes Risborough on April 28th 2015.

Whilst booked only to work the Monday to Friday peaks all day "bubble" operation did occur on a few rare occasions as did Saturday all day operation for the "Bubbles 50th birthday" celebrations on September 4th 2010.

On Sunday April 9th 2017 the Branch Line Society operated the "Bubble and Squeak Tracker" Railtour saw a sell out tour take 55034 from Aylesbury to Marylebone, H.Wycombe, Bicester depot goods line, Oxford Parkway and Oxford. From here the green bubble reversed to use the rarely used Bicester - Claydon line before returning to Aylesbury via Quainton Road. A max speed of 68mph was reached (nominal max speed is 70).

A breif return to the Mainline saw 55034 work to Old Oak Common via Paddington on August 31st 2017. After starring at the Old Oak Common Open day she returned to Aylesbury, again via Paddington on 050917


Passenger reactions

Whilst the regular commuters have become used to their heritage power comments of "what an old train" are commonplace amongst less frequent users. They then sit back to enjoy the comfy thick upholstery and ample heating. "Reminds me of a time when people didn't stab you in the back" was an overhead philosophical comment from another new "bubble" passenger.

Enthusiasts obviously enjoy travelling on the "bubbles", 55020 has earned the affectionate nickname of the "Deltic with doors" in reference to her carrying the same number as former Deltic 55020 "Nimbus"!


Chiltern bubbles finally end

The last few of regular "Bubble" operation featured 55034. By April it became normal to have several enthusiasts on some morning turns and loads on evening turns. This was all very unusual to those few who did the Bubbles from 2003-2016! The final day Friday May 19th 2017, saw 55020 resurrected and paired up with 55034 for a very busy but memorable last day in Chiltern service. The whole world seemed to be there!

The Chiltern bubble's final day, May 19th 2017


W55020 also known as the "Deltic with doors!" was introduced by Chiltern in April 2003 for use on peak-hour

Aylesbury - Princes Risborough shuttle services freeing up a 2 car 165 unit for much needed peak capacity elsewhere.

W55034 also joined Chiltern in May 2011 to supplement W55020 on occasions. The use of the blue bubble (W55020)

or the green bubble (W55034) continued on peak hour Aylesbury - Risborough branch turns on an unbroken run until

May 2017.


14030296000255020WEBR.jpg Before she was famous a departmental 960002 on Aylesbury TMD on March 14th 2002, a year before she would return as the blue bubble W55020

070403550201657PRwebr.jpg Aylesbury station sees W55020 on her inagural day in service with Chiltern back on April 7th 2003

25051155034r.jpg Thanks to Trev Rolfe for this Marsh Crossing view of 55034 Aylesbury bound on her first day in Chiltern

service back on May 25th 2011.



The final day, Friday May 19th 2017


farewellposterr.jpg Chiltern's farewell poster on display at Risborough, May 19th 2017


190517bubsar.jpg Staff such as the familiar TTI will miss the bubbles too


190517bubsbr.jpg Princes Risborough, awaiting the off with the 1450 to Aylesbury


190517bubscr.jpg Crowds are hovering at Aylesbury, some time before the pair worked the 1639 to Risboro


190517bubsdr.jpg The duo lean into the Risborough Branch curve after leaving with the 1828 to Risborough


The final day, Friday May 19th 2017 actual "Bubble" workings



55034 5P11 0708 Aylesbury - Princes Risborough

55034 2A06 0719 Princes Risborough - Aylesbury

55034 2P16 0746 Aylesbury - Princes Risborough

55034 2A12 0823 Princes Risborough - Aylesbury

55034 2P23 0849 Aylesbury - Princes Risborough

55034 2A15 0916 Princes Risborough - Aylesbury


55020/55034 2Z91 1140 Aylesbury - Princes Risborough Additional

55020/55034 2Z92 1250 Princes Risborough - Aylesbury Additional

55020/55034 2Z93 1319/2Z94 1350 cancelled due fault with 55034

55020/55034 2Z95 1419 Aylesbury - Princes Risborough Additional

55020/55034 2Z96 1450 Princes Risborough - Aylesbury Additional


55020/55034 2Z97 1639 Aylesbury - Princes Risborough Additional

55020/55034 2A46 1700 Princes Risborough - Aylesbury

55020/55034 2P65 1726 Aylesbury - Princes Risborough

55020/55034 2A50 1806 Princes Risborough - Aylesbury

55020/55034 2P70 1828 Aylesbury - Princes Risborough

55020/55034 2A55 1930 Princes Risborough - Aylesbury

55020/55034 2P76 2008 Aylesbury - Princes Risborough Additional

55020/55034 2A59 2033 Princes Risborough - Aylesbury Additional....the very end


W55020 hadn't worked for some months so it was an unexpected suprise to see the blue and original Chiltern

Bubble emerge from the depot for the final workings . This was the first and last time, as far as we know, that

the bubbles had worked in multiple for Chiltern. With the crowds of enthusiasts that turned up for the final day

the pairing was very much needed...


A huge thank you to all at Chiltern for making the last day so special for all...



Preservation is expected for W55020 and W55034 but for regulars at Aylesbury, Little Kimble and Risborough

the scenic branch will be a barren place without those warbling "bubble cars".




Pastures new, Bodmin and Crewe Oct 2017


From late May 2017 the "Bubbles" were parked up on Aylesbury TMD with just destination blind changes the only sign of movement. As mentioned earlier 55034 escaped to Old Oak via Paddington in early September 2017. Then came the final farewell with 55034 and 55020 both under power on 5T01 0808 Aylesbury - Tyseley via Risboro on October 13th 2017. Shortly afterwards 55020 went by road from Tyesley to a new career in preservation on the Bodmin and Wensford with her working services early in 2018. 55034 went by rail to Crewe DD under the new Locomotive services "TOC" banner and 55034 was out on route learning charters in April 2018, long may it continue!


The Chiltern Bubble 55020 made her first workings at Bodmin n March 2018 and has been a regular performer since then. She also starred at the September 2019 diesel gala.

She has remained in her Chiltern blue livery keeping the memory of her former local origins clear. Long may it continue!

02111955020r.jpgThanks to Neil Evans for this 55020 view at Bodmin on November 2nd 2019


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