Go to your "Settings/Options" in the game menu (Where you change the
graphics resolutions etc). Click on "Hotkeys" now look for the Action
"Cheat" and see which key in your version activates it. A dialogue box
will pop up when that key is pressed (mostly it's the Period key).
Enter "Safety First" into that box to activate it.

Now you can use the following codes:

we have a winner gold = Win the game (Gold Medal).
we have a winner silver = Win the game (Silver Medal).
we have a winner bronze = Win the game (Bronze Medal).
we have a winner = Win the game.
all is lost = Lose the game.
big dog = Give your player $10 million.
fat cat = Give your player $1 million.
bailout = Give your company $10 million.
subsidy = Give your company $1 million.
passport = Give your company access to all territories.
go go go = All trains go double speed.
oops = All trains crash.
trains are in my blood = All locomotive types available for purchase.
double shift = Buildings produce cargo at double normal rates.
safety first = Trains never crash.
upgrade = All trains upgraded to HST 125 (A nice diesel).
orca = Get the Orca engine.

Alternate cheat mode:
Press [Period], then type one of the above codes.