No, but it is listed in the number of commands. Just like the Couple command, with a little >, but unlike the Couple, when you click on Decouple, no choices are given.
To Tony, yes, the Engine is already attached to a string of cars -- however, I will go and bang it for sure, thanks for the suggestion.
Are you referring to the Decouple command itself or one of the ones on the Download Station?
The built-in Decouple command cannot be added to schedules in Surveyor as the information the game gives for that command is only present in Driver. The other Decouple commands on the Download Station should still work though.