A 50 cent Inrush Current Limiter will also cure the "zoom". This is in a Turbine but I've done the same mod in the Berk. When running along side an MTH engine set at 1 MPH the turbine will run at about 3 mph. Not bad for a conventional engine. The part number is STM CL160.
Here's a Williams Berk modified to closer resemble a Lionel Berk.
Pete