Why do Central Line trains sometimes jolt as they're leaving the station?

January 29, 2007 3:16 PM

On the London Underground, on the Central line, why does the train sometimes jolt as if it's lost power for a second as it's leaving the station?

Sometimes it happens just once, sometimes it happens two or three times. It doesn't seem to happen consistently at particular stations, and it feels like it's more likely to happen when the train is crowded (although that might just be my own confirmation bias). My hunch is that it's a safety feature of some kind, but if so I can't figure out what might be triggering it. Anybody in the know?

posted by chrismear to travel & transportation (9 comments total)

For a number of reasons, including safety, electric subway systems are divided into multiple power sections, each with its own transformer to supply power. When a train moves from one to another there will be a small discontinuity, as it coasts over the break between them. I've never been to London, but the Boston system exhibits the same behavior and that's the reason.

It also happens in train lines where part of the line is powered by a third rail and part by an overhead wire, such as Boston's Blue Line. A gap in power can take place during the transition from third rail to overhead wire.
posted by Steven C. Den Beste at 3:40 PM on January 29, 2007

I always thought it happened when people were leaning on the doors too much. I don't have any evidence to support such an assumption though...
posted by ob at 3:55 PM on January 29, 2007

I once made a film about people dragged to their death under tube trains. I think I remember that the live rail is in a different location (on the side of the track) inside stations than it is inside tunnels. So the jerk might be a transfer from one rail to the next.

Yes, it was a pretty ghastly project.
posted by unSane at 3:58 PM on January 29, 2007

LU uses a four-rail system, not a three-rail system. Electricity is transmitted both through the middle rail and the rail on the side of the track. I don't think this changes in stations, but it is different from many metro systems, so that may be what unSane is remembering.

A peculiarity of the Central Line is that trains run automatically between stations. This happens on the Victoria line as well, but with an older system. You may be experiencing a shift between manual and automatic control. That would be my first guess anyway.

If you want a reliable answer, ask on the newsgroups uk.transport.london and uk.railway which are full of signalling geeks, real tube train drivers, and other TfL/railway employees.
posted by grouse at 4:21 PM on January 29, 2007

Here's your question answered by actual London Underground employees. Note the jargon:

CTBC = Main speed control lever
Section gap = what Steven said
Pilot light = Indicates doors are all properly closed (etc). Losing it immediately cuts power to the motors.
Oxo = Oxford Circus

You may be experiencing a shift between manual and automatic control. That would be my first guess anyway.

That would surely only be done when the train is stationary.

I don't think this changes in stations

It does change sides in stations so that the live rail is on the side furthest from the platform. The two sides overlap when they change sides though (unless there's a section gap), so there wouldn't be a cut in power.
posted by cillit bang at 4:47 PM on January 29, 2007 [1 favorite]

I would guess it's not a loss of power but a loss of traction. Metal wheels don't have great grip on metal rails.
posted by randomstriker at 5:06 PM on January 29, 2007

It's when the emergency stop kicks in - people lean on the doors and the train accelerates, the doors open slightly which triggers the emergency stop.

(A tube driver told me this is why, and I experience it most mornings!).
posted by goo at 12:36 AM on January 30, 2007

It's people leaning on the doors. Sometimes a driver will mention it over the PA; I heard one smooth-talking driver on the Central line explain that "leaning on the doors may be detrimental to your journey, as you have discovered", while last week on the District line, after about five such jolts as we left East Putney, came "will the BLOODY IDIOT who is leaning on the doors stop NOW!". But generally the jolt seems to shift people off the doors, so they never have to mention it.
posted by terrynutkins at 2:00 AM on January 30, 2007

It is most definitely the emergency stop, triggered when people lean against the doors.

The driver will sometimes explain this if he's in a particularly bad mood.

/longtime Central Line commuter
posted by bifter at 2:20 AM on January 30, 2007