One error in transport tycoon is that train depots, in terms of signalling are considered to be continuos pieces of track. This leads to some interesting things:

Firstly you can trap a competitors train in its depot. Build tracks up to the back of the depot and build the cheapest possible train. Run the train out onto the tracks then stop it. You will have to click the "Ignore Signal" button to make it come out of the depot. Then you have the following:

Now, the next time the competitors train goes into a depot for a service, it will never come out again, because the "depot signal" will not read clear since your train is sitting on what the game thinks is the same segment of track:

This is brilliant because it doesn't make any profit and because it still has to pay running costs the competitor makes a loss. Your only cost is the depreciation value of the train you bought.

This works because when the train enters the section of track leading up to the station, as far as the game is concerned the whole section of track round the back of the depot and the stations is occupied, so the moment a train tries to enter the station, the lights turn red to stop it, and the joke is, that the lights are turned red by the train itself! This cheat causes no cost to you except for laying the tracks