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Underground-Symbol-Small.gif (1234 bytes)Tubeprune - The Tube Professionals' Rumour Network
Tubeprune is an unofficial web site for professional railway people working for London Underground and for those interested in the London Underground railway system.  To the Tubeprune Home Page

London Underground - Frequently Asked Questions

Questions and answers to how and why things are done the way they are on London Underground.  Many come from messages posted to the newsgroup  See also Forum for the answers to many questions.

Why Four Rails?

Q. What's really questioning me is, why they use a positive (+420V) and a negative (-210V) rail? AFAIK LUL has the only system world-wide with two current rails.  One current rail providing +630V would have been enough with tracks providing earth, wouldn't it?  All other systems work that way.    Has it got to do with the tunnel segments which IIRC contain steel which would then be live?  Then again why -210V? from Christopher Guy Perry 1 March 2002.

A. See: Why 4 Rails? on the Track and Traction Current page.

Air Conditioning?

Q. Why don't the trains on London Underground have air conditioning?

A. A number of reasons.  First, the climate in the UK is not generally warm enough to warrant the expense of installing air conditioning in houses and it is only put into modern buildings where glass walls increase the ambient temperature.  On the Underground, if air conditioning was put onto trains, the waste heat would be dumped into the tunnels.  There is nowhere for it to go, so the tunnel temperature would rise even more than it does now.  The control of tunnel temperature is a constant battle requiring ventilation plant all over the system and which is combined with draught limitation shafts to keep the wind speeds low enough for comfort and temperatures at a reasonable level.  If trains had air conditioning, then it would soon be necessary to air condition the stations and tunnels.  This would be excessively expensive.

On tube trains, which are smaller than the District, Circle and Metropolitan trains, there is no room to put air conditioning equipment unless you enclose several square metres of passenger space inside each of the cars.  Also, the energy consumption of air conditioning is very high and there is not enough voltage available over the system to support air conditioning without increasing the capacity of the power supply system.


Q (From "Keston" u.t.l 26 April 2001).  Why can't LU increase the number of trains on the Central line outside the peaks?......Why don't they just increase frequencies during these off-peak times where there is definitely demand for a couple of extra tph?  Driver shortages?

A. To answer the last question first, yes.  Off peak services are being severely hit by driver shortages.  They always are when the economy is booming, even though LU currently offer about 28,000 a year.  If we go into the predicted recession, watch the driver recruitment improve.   One Saturday in April 2001, 150 train trips were cancelled due to no driver available.  Still, it better than it was in the early 1970s.  Then, less than half the scheduled service operated on most lines due to staff shortages.

London Underground is chronically overcrowded.  Most of the lines in the central area have a system capacity of about 12,000 - 19,000 passengers per hour per peak direction.  All are running over this (e.g the District is at 16,000 at the lower end of the scale and the Victoria is at the top with 28,000) and there is no way to improve the capacity substantially unless extra lines are built.   The PPP upgrades will only tinker with the system in terms of capacity.   They might get 10% more out of the system.   Money should be found for the Crossrail and Chelsea - Hackney Lines to be started at once.

Q. Also - why can't they run more trains on Friday and Saturday nights?

A. LU can't get enough drivers for the existing timetable, let alone a more frequent one.

What is that humming noise?

Q (From "Lew " u.t.l 24 April 2001). Why do older tubes hum like they do. I've wanted to know since travelling on my first old District Line train and no-one, not even an electrician father-in-law-that-wasn't of mine, knows this.

A. The hum is from the motor alternator (known as the MA).   This is a machine which provides low voltage supplies for lights and control circuits like door open and close circuits.  You will find it under motor cars, not trailer cars, except for the 1967/72 fleets.  If you notice a change in pitch of hum, it is due to the fact that the motor is powered off the traction current supply and this varies as nearby trains accelerate.  I can remember as a driver, often sitting waiting at a red signal in the cab listening to the hum of the MA.   When the pitch dropped, it was usually a sign that the train in front was on the move at last and the signal would shortly clear.

Stock built in the 1990s (Central, Jubilee and Northern Lines) doesn't have them.  They have "auxiliary inverters" instead.   These also hum but not loud enough to hear because they have no rotating parts.

The rattling of the windows you can hear on some stocks is due to the air compressor.  This is a pump driven off the traction supply.   Compressors switch in and out as required to keep the train's air supply up to the required level.  You often hear them running at stations after the train has stopped because it has used up lots of air for the brakes and doors.

How does the Tube cope with snow?

Q from Dave in u.t.l, 21 January 2001.

A. Some sections use heated conductor rails at night by upping the voltage.  A number of passenger cars are equipped with tanks of de-icing fluid and sprays which are used as the trains run in service and which are also run at night without passengers.  The cars have blue indicator lights at roof level, like the yellow door fault lights.  These trains are also equipped with sleet brushes, which can be lowered by closing a switch in the cab.

Some sections of track have de-icing baths set into the rails.   The collector shoes of the trains pick up de-icing fluid from rollers fitted in the baths and spread it along the rails. 


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