How to reverse a motor polarity using reed switches

[tigertankcommander]

I want to build a model cable car that runs one way then when the cars get to the end they activate reed switches so it reverses the motor polarity so they return back again and just keep doing that

 

[rjenkinsgb]

Use a relay to control power, eg. a three or four pole changeover type.

Two of the changeover contacts form a reversing switch setup to feed the motor.

Another contact can be used as a latching contact for the coil, switched on by a normally open reed switch contact and off by a normally closed (Form C) one.

Add a flywheel diode across the relay coil to minimise the back EMF; reed contacts are extremely sensitive to spikes and high currents.

edit - examples:
The latching circuit; the redd contacts replace the push buttons, the relay coil voltage needs to match the supply you are using:

http://www.reuk.co.uk/OtherImages/make-latching-relay-with-a-dpst-relay.jpg

The reversing part, using extra contacts on the same relay:

http://qph.fs.quoracdn.net/main-qimg-2b20f70f890ffbc0f6d6d7723bef98bb