When money gets to about £2billion, it flips over into negative. However, this means if you build something costing more than £2billion it will earn you money, such as a very very long tunnel. However, it is not pleasant to have a successful company with more than £2billion in capital suddenly to go bankrupt because of an error, so a fix has been introduced recently so this may not work. Any way to get money press shift then go to the edge of the map and look to build tunnels, build one where it says you will get an income:

It is not advisable to do this more than once because it could flip your cash over into negative. However in later versions of transport tycoon the total cost of tunnels is limited to about £400,000,000 which eliminates this game.