It's an overflow. The maximum value you can store in a 32-bit variable is 4,294,967,296, and the variable used to store your money is 32-bit. As you can have negative amounts of cash, the value range is -£2,147,483,648 to £2,147,483,648, and anything above that will become negative.
I don't have time to look at everything you know
With some topics locking me for 5 minutes because someone has a bad avatar and other topics filled with some mambo jambo
Seriously, download TTD and use the patch, there is nothing but advantages to that
Well, back to work, lot's of it in the near future
I don't cheat. At the start of all my games, I pay off my loan and use cht: money 1000000 and build a train line (ONE train line). Then I use cht: money 0 and wait for cash
Sounds like cheating to me. Why don't you simply build a train line using the £100,000 available to you at the beginning, borrowing more money if necessary? (Unless you've set your loan max to £100,000).
Within a few years, you'll have paid off your loan and will be raking in the money. Personally, I'd rather pay loan interest for a few years than cheat... but that's just me, you may not mind about cheating.