| MA | Postal Car constructed and equipped for passenger train service, with side doors, with or without mail-bag hook and suitable fittings for sorting and classifying mail in transit, with or without end doors and windows. |
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| MB | Baggage & Mail Car constructed and equipped for passenger train service, with side doors, with or without mail-bag hook and suitable fittings for sorting and classifying mail in transit, with or without end doors and windows. Having two compartments separated by a bulkhead and having suitable side doors. |
| MR | Mail storage Car constructed and equipped for passenger train service, suitable for carrying mail in bulk, and without suitable fittings for sorting and classifying mail in transit, with or without end doors and windows. |