Set default service interval for new engines to the given number of days.
Configuration file: servint
Command line: -i
If you think that the train behaviour is annoying when it comes to service, because they keep going the wrong way looking for a depot, just set "Vehicle Breakdowns" to "None" in the difficulty settings in TTD, and enable this option. This will set the default service interval to 50 years, and no train will ever try to enter a depot again. You might also want to have a look at the Cht: ServInt sign cheat (see Sign Cheats).
You can give a different number to set all default intervals to 90 days for example, if you'd rather have them go to the depot more often.
Note that this only works for new engines that are bought while the switch is enabled. Use the sign cheat to modify already existing engines.
An alternative solution is to use the gotodepot switch; see Go To Depot.