2756 is a Milwaukee Road "branchline caboose". It was one of ten (2750-2759) built in 1938 - note the car siding which is derived from the look of their flagship "Hiawatha" train. When MTM acquired it in 1990, it was marked as X405 - the maintenance-of-way number it received just prior to retirement. MTM returned it to its original number.
The inside was stripped prior to acquisition by MTM, but divisions between coach, baggage, and RPO sections are still clearly visible. Also visible are the ceiling vents for dual-burner gas lights.
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