I would like opportunity to be able to sell the bonus engines if not required anymore. now that we are in era 5, i have a engine that cost more in maintenance than it makes. i know you can upgrade it, but i must doing something wrong as the game would need 20 era for me to upgrade it.
If it is costing you more to service than it makes, try using the timetable calculator to get more profit. You should be able to find a facility supply run that will keep it profitable for at least 12 hours and just periodically change schedules. If you still can't get over the servicing cost you can always park it in the city so it takes no damage.
EDIT - the same is true for low level career engines. I am able to keep my ERA 1 goods career engine with only 20% reliabilty profitable in Era 6, by simply updating its schedule every time I log in.
park it in a city; parking trains do not need maintenance...