That is the 1957 PA/PA Northern Pacific set and is very desirable. The 490 is a dual motor unit, the 492 is unpowered and contains the diesel roar unit and the diesel horn. The value will vary widely with condition, if the roar and horn are operable, if the horn controller is included and if the powered chassis runs as new with no bushing wear. The OB's with a legible number stamp will add $50 to $100 to the value.
If you are not familiar with Gilbert diesels and how to determine the condition of all the components this can be a risky purchase because they are relatively expensive.
If these are a full excellent condition with no bushing wear, operable accessories they are worth at least $400, probably $500 IN FULL "E" CONDITION. Value falls with poorer condition, bushing wear or inoperable roar and horn.