Are you talking about the distance you need to be behind another vehicle? If so, for a bus it's 4 seconds. What you do is look at the vehicle in front of you and when its backside passes a sign, you need to be 4 seconds behind that, when you reach the sign, more in inclement weather.
That is different than what is traditionally taught for normal size vehicles which have either the 2 second rule or 2 vehicle lengths behind the person in front.
As far as trains go, it was the one mile rule or if you could see/hear it you had no business going near it. The number of cars and weight can vary on trains, so it would be difficult to determine by that.
Here's a bit more info about them: