From #openttdcoop wiki
A Sandbox Scenario is a simple scenario that provides lots of money and empty space where you can experiment with building a transportation network. An empty scenario with a single town (population 0) and no additional GRFs selected is about 10k in size.
How to create your own Sandbox Scenario
- Start OpenTTD
- Configure the NewGRF Settings you want your sandbox to use. Specifically, you'll want to select the train set you wish to use for your testing.
- Click the Scenario Editor button
- Click the Landscape Generation button and then select Create New Scenario
- Determine the landscape, size, starting date, and height of the land to your preferred values. Typically, you'll want at least 128x128, starting late game (for more train availability), and use a height of 1.
- Click the Flat Land button to generate a totally flat landscape
- Click the Town Generation button
- Click New Town, and place a town anywhere on the map (a town is required to load a scenario)
- Use the dynamite tool (on the Landscape Generation toolbar) to destroy the entire town.
- (Optional) Rename the town. "Sandbox" is a good name for it.
- (Optional) Place a sign to remind you how to do the money cheat (see below)
- Save the scenario with a meaningful name (again, "Sandbox" is a good choice. If you have multiple sandboxes for different train sets, you might also include the trainset name)
How to run your own local Sandbox
- Start OpenTTD
- Click the Play Scenario button and select the Sandbox scenario you created earlier
- Once the game loads, access the cheat menu through Ctrl-Alt-C (or Ctrl-Alt-Win-C)
- Click the right arrow next to the first option to receive 20M in cash
- Experiment to your heart's content.
- Since the scenario file is not edited when you save a game, you always have a clean slate to start on (just remember to use the money cheat on later games)
Pre-made empty Sandbox Scenario
If you just want an empty sandbox scenario using the default GRFs, you can download one here