Attach up to 126 wagons to a train.
Configuration file: mammothtrains
Command line: -z
This switch allows you to have very long trains, up to 127 cars. You will need EnhancedGraphicalUserInterface on to be able to see more than ten of the cars attached to a train. Trains that are quite long will obviously have difficulty getting uphill, so it is probably a good idea to enable MultiheadedEngines as well.
If you use this switch, there is no real limit to how many cars your trains can have. Well, there is a limit, but 127 cars is just so much that it won't matter. Really. Trust me. Especially since it would take ages to buy such a train...
This is meant to be able to fully use length five (or seven) stations with the above switch. Now you can have a double-headed engine and ten cars and still load it without penalty. If you intend to buy really long trains, you should really enable the improved load time calculation or the train will be in the station forever.
You will still only see a limited number of cars in the depot. With enhancegui you'll see 14 or 18 wagons and engines, depending on the setting. Without enhancegui, you'll see ten wagons as usual. This means that if you want to rearrange your train, you'll have to work in reverse, adding cars at the front of the trains.
The depot also behaves slightly different, because when a new train only used to attach at most nine cars to it, now it will take all that are available (up to 127). This means that you shouldn't try to upgrade two trains at the same time, because one of them might get all the cars, which could cause a headache.
Another thing is that this can make the layout process of a station very challenging. Even though you may not get a penalty (or at least not as much) for a bit of train sticking out of the station, the cars will still be there and block tracks and cause signals to remain red. If you don't leave enough space at the entrance of a station, there will be a major traffic jam.
And, most importantly, if you have a really long train, try not to make it go atop any mountains. Although, surprisingly, even a fully loaded 126 car train, at a total weight of around 7000 tons, made it. It was only moving with a few MPH, but it's still amazing (and not very realistic) that it made it eventually. However, if you're using realistic train acceleration, it is possible to make a train that is too heavy and will take ages to move (see New Curve and Mountain Handling).
So, I suggest you don't go over about 20 cars or so, except maybe for a bit of fun. You can even make a train go through itself without crashing...