Welcome GWRGirl

Hi Gloria
I suspect that your loco has a pickup issue as you said the motor starts to go then stops if it's th Bachmann small small prairie they have wipers that contact the rear of the wheels often they are poorly fitted and fail to contact properly you can lift the wheel Se t out and they can be eased out so you have even pressure on both sides of each wheel set also check that when the side play in the wheel sets are to one side they still contact or it will have issues negotiating curves.
It's also possible that there is dirt build up on the pickups.

If it's the Hornby large Prairie sorry I'm not sure how they pick up haven't had one apart however possibly a similar wiper pick up.

Sorry I'm a bit far away to look at it for you.
Good luck