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I have 27 15 year old N scale diesels (1996) that are not DDC friendly.
I'm thinking about going DCC, but cost & the trouble retrofitting these are killers.
Other than Aztec & Souther Digital, are there any other companies that make frames for these engines?
The only other choice is to modify the frames your self with a hack saw and a file. IF they are Atlas or Kato engines, check to see if they have light boards in them. If they do TCS may have a decoder that will be less trouble.
If you are planning to use code 55 track, you need to check to see how deep the flanges are on your old locomotives. I have had to put my old engines out to pasture due to changing to code 55 track.