I bought and set up a Williams BL-2 diesel locomotive on my 40x80 layout of MTH Real Trax,  powered by a MTH Z-1000 controller/transformer (18 VAC, 100 watts).  The locomotive runs fine, but the headlamp barely lights at all.   If I add any type of lighted or illuminated cars I get the same result (bulbs are very dim).  If I increase the throttle significantly, the headlight brightens just a bit, but the locomotive's increased speed will send it off the track.  I don't know why this is happening. 

I thought the Williams locomotives would run properly with any standard transformer.