I`m guessing that by stopping after 12 cars you mean that the industry runs out and has to wait til it has more to continue. I`m using the coal mine installed in trs2006 to load a coal train of 50 cars and all I did was go into properties and set the production rate high enough to keep up. It means that the coal mine is pretty much full to capacity most of the time but it allows me to run the length of train I want. I just multiplied to cars capacity by the number of cars divided by how many minutes it takes to fill 50 cars to get how much coal is going out in 1 minute while filling. For the time I just timed how long 10 cars took to go through multiplied by 5. Takes a bit of time but its worth it. The only problem I had was I had to use a MIN track to unload at the power station because the train was carrying more than the power stations max capacity and therefore wouldn`t unload completely.