Running MTH 2/3 rail Locos - Caution

[wrmcclellan]

One of our club members brought his new MTH 2/3 rail SF Pacific to the club layout a couple of weeks ago and we had terrible problems with the loco arcing on its wheels as it went around the layout. The arcing is serious enough to cause pits in the flanges and tires.

We discovered the problem.

As many in the hobby do, we use one of the outside rails for signalling. Normal 3-rail locos have the wheel sets electrically connected and thus can be used to ground/short a signalling or accessory activation control rail to the AC common rail (other outside rail which is connected to the transformer common terminal).

In our case we have a small 24 volt AC transformer to supply current to the signaling relays.

When the new MTH loco is switched for 3 rail, this apparently does not change the fact that the wheel sets are still insulated from each other. Thus when the loco is on a track with outside rail signaling, the control rail voltage (which is no longer being connected to AC common through the wheel sets) is now applied to the insulated wheel and thus - depending on the loco wiring - to the loco frame (which is normally common) or other electronics.

I do not have any more specifics on what was arcing to what as this would mean some time electrically measuring the loco in question which we did not have that evening.

The good news is we have only applied outside rail signalling to one main line out of three, so we simply moved the loco to another main line and "banished" it from the line with signalling.

It will be interesting to hear of other's experiences with these new 2/3 rail products.

Regards,
Roy