The default in game currency is Dollar $ . All in game values will be displayed as "K", "M" and "B".
If you want another currency or want the value in fully written form then this little how to can show you the way:
The settings file for the currencs is located here:
1. Train Fever must be closed!
2. Find the line where the following value is located:
|moneyPrefix = "$",|
and switch it to the currency you want
|moneyPrefix = "€ ",|
3. Find the line where the following value is located:
|moneySuffix = "",|
and switch it to the value you want
|moneySuffix = " EUR",|
4. Save the file.
5. Check it out.