You could try using the Geldkutche mod.
I use v1.0. You just place one (or more) vehicles in a depot. Each vehicle costs $0 to buy, and has an annul running cost of -$1,000,000 giving you an extra $1,000,000 income per year for each vehicle. You don’t need to set the vehicle to a line, just leave them in a depot. Best to build a separate depot for them in an out of the way corner of the map.
As this does does not modify the game in any way, you can play in the normal way until you run out of money, then buy as may vehicles as necessary to get you out of trouble. If you get too much money just sell the vehicles.
You can find the mod here: http://www.train-fever.net/filebase/index.php/Entry/328-Geldkutsche/