Old thread,but I will add few comments here since I have some knowledge and expertise with electrical railway infrastructure.Max flashover distance of MV power lines can be pretty good judged byy the lenght of insulators.
OHL support insulators for 25 kv @ 50 Hz railway should be at least 400 mm long (linear lenght,outdoors).Here you can see how typical railway insulators look like:
http://img267.imageshack.us/img267/4025/p3010003io5.jpgAny gap shorter than insulator's lenght is in danger of being closed by arc if the sufficiently high overvoltage arise in the system.In comparision with ordinary 3-phase MV utility lines ,25 kV railway lines are more dangerous ragarding these matters.
Play safe, always respect minimum approach distance rules.And even if you play strictly by rules you're not 100% safe.Freak accidents are statistical rarity but reality too...
