many Marx motors are swappable and not all will fail on modern track with guardrails. find a #666, #1666 or #400 locomotive with what Marx collectors refer to as double reduction gearing. i have a few CV, Mercury and M10005 engines that i have modified to run on all 3 rail track. there are also rare versions of the #999 and CP types that were made with DR motors. the only 6-wheel motor Marx made, the #333, is compatible with all 3 rail track, too.