Our family used to go to the Exposition every year, from the late 50s through most of the 60s. I remember the New Haven Railroad's display well and have 3 of the "Honorary Engineer" cards that were given out on the locomotive. Most years featured a shiny FL9 with stairways in and out, and with some foam rubber taped over the low locomotive doorways to prevent head injuries. One year had an RS3 and primarily freight equipment, and one year had an EF4 "Brick" electric loco. I believe the last year for the New Haven display was 1967, maybe 1966. I remember going over to the display area (behind the Young Building) and just finding a couple of boxcars with partial loads of hay the year after the final NHRR display.
When Amtrak was introducing the new Amfleet cars in 1975, they brought over a coach, a cafe and a baggage car with a diesel generator inside for "head end power" for the passenger cars. I helped staff the display, which was located east of the Young Building instead of where the NHRR display had been. I think there was only one year for the Amtrak display, but in any event the siding was removed shortly afterward and with it the likelyhood of displaying any full sized railroad equipment.
The only other full sized rail equipment outside of MoW that I remember was a narrow gauge steam locomotive on the lawn of the Mass. state building during an Edaville revival attempt during the 1990s.