I wanted to add to Don's advice about using accessory decoders for
stall type motors. That is true. However, you can't use just any
relay. Check the maximum current the decoder can put out for a stall
type motor. Then make sure you relay does not require more than that.
If there is a relay you want to use, you may have to shop around for a
stall type motor accessory decoder that can put out as much current as
you need. I know that some of the accessory decoders can only put out
40mA or less. Some relays require about 160mA.
There is also a type of relay called a "high sensitivity relay."
These relays are specially designed to work with computer type
equipment and draw much less than the typical relay. They cost a
little more than the typical relay, but not too much.
Be careful. I'm talking about the coil sensitivity rating of the
relay NOT the current carrying capability of the relay CONTACTS. Your
coil sensitivity rating should be about 160mA or less (especially for
a high sensitivity relay) whereas the CONTACT rating should be about