[Daem]

OnGameModeInt
{
ConnectNPC("TRAIN_DRIVER_NAME_HERE_IN_OTHER_WORDS" , "FILTER_SCRIPT_HERE_INS'T_IT_?");
return 1;
}
OnPlayerConnect(playerid)
{
if(IsPlayerNPC(playerid))
{
PutPlayerInVehicle(playerid, YOUR_TRAIN);
}
return 1;
}

you can also check V1ceC1ty's NPCs scripts it can explain everything
or
http://forum.sa-mp.com/index.php?topic=119947.0