Ed,
what you really want to monitor is the amperage. You can see the effect on the voltage by adding the lighted passenger cars. (higher amperage required)
Does the 14v stay constant when you put the train motion?
The ZW-L has some dynamic power handling feature. Bump the voltage back up once you have the whole train on the track.
You should be able to roll it up higher than 14v with that load on it.
I would say that it is expected unless you have the throttle at 100% on the transformer. The ZW-L can deliver enough current to run that train at 17 or 18 volts.
If the car lights get too bright, run in at a lower voltage.
