MLW 6 axle M Liners (M630 M636 M640) - MagnumMacKivler/RLCPT2 GitHub Wiki
Though the M-Liners were produced in small quantities, however, there were a few phases.
Ph1a: These units were the first built, and look very much like a C636 sans air intake. They have an air start (three airtanks) and an exhaust with a boxy housing.
Ph1b: These still have air starts (three airtanks), but have dropped the boxy housing around the exhaust.
Ph1c: The majority of M-Liners were built like this, they have an electric start (two airtanks) and the more streamlined exhaust housing.