The 'communication' is by a 36-way jumper cable which runs throughout the train, connected at the end of each coach. As with anything electrical, the signals travel very quickly but, yes, eventually over a very long distance they would potentially become prone to reliability issues, bearing in mind we are not talking hi tech kit or high voltages here. That distance would need to be far greater than even a very long train though I would think. More likely to cause interference would be an excessive number of jumper connections, which on Mk3's have been known to prove particularly vulnerable to damage from debris and obstacles. It's not unusual for the CDL (door locking) to develop jumpef-related faults, and at one time it was fairly common for the crew to have to go lineside and reattach the train supply between coaches, again usually due to them being swiped by something on the track. Spares are usually carried in the power cars.
