Is your Mohawk the 18009, or 18064? The 18009 is very easy to work on; it's easy to remove the boiler, and under the hood it's no different than any postwar, or even prewar Lionel. To restore original performance, just clean out any hardened grease, carefully re-lubricate, and let 'er rip!
If you bought the 18064, it was equipped with command control from the factory so there will be circuit board(s) inside the boiler, and electrically isolated handrails. An issue came to light after it was released, that the motor driver board tends to starve the big AC motor for power. If that's your issue, you'll have to locate the updated replacement board, or install some other type of control system altogether. In either case you should be able to get your new train running yourself, and enjoy it for many more years.
