The food costs itself should be controlled by managing the vendor more closely. A beer costs $3.50 on AMTRAK, and at most stations, a beer can be had for $1.00 from many vendors.
The labor cost is another matter. Perhaps AMTRAK should void the Gate Gourmet contract and put the whole thing -- material plus labor out to bid. AMTRAK provides the dining or cafe car with utilities, the vendor provides everything else. I bet that if the vendor were allowed to keep whatever profit they could make, there would be several competitive bids.