With the new information about the caboose and the great magnifying glass examination by MTN the only possible starter catalogued set would be the the 1957 20320 Mountaneer. It contained the 925 Gulf tank car, 937 MKT box car, 928 New Haven lumber car and the 979 Action caboose. The engine was the C&NW Pacific. Since yours appears to have the die cast trailing truck it would be a 21084, indicating a late production set. The 21084 was an uncatalogued engine packed in some of the 20320 sets. The more common engine in that set was the 283, but it did not have the die cast trailing truck.
The cars above would have been in blue/yellow boxes in the set box. The other cars if purchased in 1958 or later to add to the set would have been in red/white boxes. This set also included a 50 watt transformer. Someone decided the larger Lionel transformer was better.
A major caveat is Gilbert made hundreds of uncatalogued sets, some with as many as 13 cars. Robert Tufts published a book detailing these. It is not computer searchable so I have not been able to quickly see if one of those is a closer match to your train set.
All the Gilbert catalogs are posted online at www.myflyertrains.org if you want to see the catalog listing.
