Fairly simple to reproduce.
1. make a really long and flat map, say 2048x16.
2. dig down short edges so that a tunnel can be built
3. build the more expensive maglev tunnel from end to end
4. You should now have positive income equal to the build cost of that tunnel
Destroying the tunnel will deduct a lot more than usual.
I haven't noticed it with other tunnels, so I think it might be some kind of integer overflow.
I think that it is probably an integer overflow, too. Is this situation ever likely to arise in a real game...?
it is exploitable, to an extent.
The amount of interest earned off of the profits is roughly 1/10 of the maintenance of the tunnel, so it can take a while for all the money to be drained.
The best use I can think of is paying for a small construction project with a separate company, so that it's free to an existing company.
Very similar bug used to be in TTD - building a bridge across the map earned you a huge amount of money :)
Found and fixed on the 11.x branch.