Simple way to unlock new vehicles whilst in Free Game

1. In the menu click Free Game
2. Let a map generate
3. Click custom and click settings
4. Make sure the only mod selected is sandbox
5. Click Save
6. Click Start

Once you're in the map, go to the year in the bottom right corner. If you keep changing the year back and forth you will unlock new vehicles.

If you enter 5000 which is the highest year, you will unlock more vehicles and also obtain the achievement Train Fever which is reach the year 2050 using only trains.

Keep changing the year to obtain new vehicles. Especially if you're in the year 1850.