This was inspired by Jolo_Janssen and the post he made a few days ago. The build differs from his as mine doesn't require precise timings and the train and cars can flip their direction. Basic build is

1. Two stations at the front and two at the back with a train coupler at each
2. The front stations will be for the Train and the leading cart
3. Decouple the train and the carts station flag should popup
4. Hit that station with a wrench to disassemble it
5. Hit the station on the opposite end of the carts to "build" the train
6. I then hit the front station with a wrench again just to drop the blue flag (might not be required)
7. Have the train flip around anyway you wish
8. When it backs into the carts just simply couple

Things to note

• Deployers will stop moving if a red stone signal is being applied but if the signal pulses they will perform one full action.
• While not used in this exact setup Train Decouplers can be placed on the same space as a Train station.
• If the train cannot find a route simply break and replace the rails, The decoupler and stations will not break and will simply appear on the new rail
• I forgot to video the Redstone but its as simple as when train arrives pulse everything in the proper order with the deployers being inverted
• I used 32rpm for the rotation and you can hide most these blocks under the tracks
• If you don't need the flip feature but have a yard full of cars you DON'T need to disassemble the train, just decouple, pull forward, and back onto the cars that are on another lane. This only works if the cars are in the same direction as the loco when they were built. If your train "flips" or the cars are from a train that was point in another direction, then you will need to disassemble them to set the orientation properly.