That's pretty common - check the wiring - are all the wires attached? If they're all attached properly, the next step is to make sure the lever for the E-unit is in the correct place so the reverse unit cycles - the E-unit can be locked out so the engine only runs in one direction. Is the engine running forward only, and you don't hear the E-unit cycling (or trying to cycle)? Slide the lever to the opposite side and see if the reverse unit begins cycling. Is the engine going through the forward-neutral-(reverse)-neutral-forward-neutral cycle?
If moving the lever for the E-unit doesn't pan out, there's a strong likelihood that the problem is one of the following: the E-unit fingers are gummed up, the fingers aren't all making proper contact on the reverse unit's drum, the drum isn't rotating properly, or there's a problem with the drum itself (track voltage left on high, and the plastic distorts or melts a bit from the fingers getting too hot).
First suggestion would be to spray some plastic compatible contact cleaner on the drum and finger assemblies. CRC QD Electrical Grade contact cleaner works well - Home Depot carries it in their electronic supply section, and it's also available online from a number of vendors. Make sure you get the cleaner that's plastic compatible - the E-unit drum can become damaged if you use a contact cleaner that's meant for metal items only. Spray the fingers and drum, let it dry (doesn't take too long) and hopefully that works for you. If not, try one more spray of the contact cleaner and cross your fingers.
If those don't do the trick, you're probably going to have to disassemble the reverse unit and replace the fingers and/or the drum. If you're uncomfortable doing that, PM me - I've been repairing trains for about 25 years and have made E-unit repairs a number of times.
