[Source: The uk.railway FAQ (www.railinfo.freeserve.co.uk/faq/)]

You should never touch the third rail or an overhead lines. Contact 
with the live rail or overhead line will result in severe burns or 
death.   

It is theoretically possible to stand on the third rail, as long as no 
part of your body provides a earth by touching the ground or another 
object. However do NOT try this out at home! Anything in contact with 
the third rail is also dangerous for example a puddle. You always 
assume that the third rail and overhead lines are live at all times.   

Live rails are very unsafe. A few years ago a set of regulations called 
the Electricity at Work Regulations came into force. They state, 
amongst many other things, that all live conductors must be insulated 
at all times. Exceptions to these regulations have to be issued 
individually by the Health and Safety Executive. The last time I 
checked up there were only three exceptions to these regulations, one 
of them for live rails on the railways. The fact that there are so few 
exceptions indicates how dangerous live conductors are.   

It is not possible to install a new live rail system in the UK, unless 
it is insulated. It is however permissible to extend an existing 
system, for example lines to Chester and Weymouth. An example of this 
is in Newcastle.  

This used to have lines electrified with third rails which were removed 
in the 1960s. It was not possible to equip the Metro with a third rail 
system when that was introduced in the 1980s. The live rails had been 
removed and could not be put back.   

Overhead wires are generally at 25kV so you will be electrocuted if you 
stand closer than whatever the flashover distance is. This is why 
rolling stock has an orange line painted on, it is not safe to move 
higher than this line while under OLE.   

To make it clear never, never touch any third or fourth rail or 
overhead lines at any time.