My guess would be that they were on the move to replace locomotives. In the early 90's Conrail still had a large handfull of GP-8's, GP-9's, GP-10's, GP-20M's, GP-30's, and GP-35's along with a couple U-Boats U23B & C's, U23B & C's, U33B & C 's and U28B & C's. still in there fleets all were retired by 1994-1995. Many of the Geep's sat in dead lines from 1985-1990 and went back into active service around 1990 as for the reason for the sudden reactivation of these units I have no idea maybe a large increase in traffic called for them?
At least 6 U33C's remained on the Conrail Rostor well befor the CSX-NS Merger 3 were keeped in Buffalo and 3 in Albany as Heavy Yard Pullers. I do beleve that all were scraped befor the merger. This explanes the missing gap 6600-6605 number gap this was the U33C fleet that was keeped off the rostor but stayed well hidden in the yards. Rember the C30-7A fleet started at 6606.