TrainFam: Well you are right indeed! The "8994" mystery is upon us...Here are a few things I found:
1. Marx was particular about his set numbering system (even if to us collectors it seems he wasn't particular about much else) and the number 8994 DOES fall in the sequence 8990-8999 which Marx designated for sets with the 898 engine. So the engine is most likely correct for the set number.
2. As you have found out before set number 8994 is indeed at least partially (Marx did use some numbers more than once) assigned to a freight set...in fact its assigned to two ! The following information is from Greenberg's Guide to Marx trains, Volume III, Sets, 1991.
8994 (A) Steam Freight Set with a CP type locomotive, black with a silver pilot, 951 Black wedge NYC tender, 59 UP Cattle car, 552 CRI&P Green (also reported as Blue) gondola, 553 SF Tank Car-silver,556 NYC caboose-red.
8994 (B) 898 0-4-0 loco, black, reverse, lighted, 551 Black NYC tender with rivets, 552 CRI&P gondola-blue, 553 ST tank - silver, 556 NYC caboose - red.
Cars have tab / slot couplers. Set includes track and transformer (Watts not specified in this reference)
Now to me part of the mystery is that this reference does NOT seem to show the 1955 passenger set from the Sears Christmas Book, regardless of whether the cars are the red/cream Bogota or the silver NYC, as that set very clearly DOES exist. In fact I cannot find a passenger set listed with any set number with the 898 electrical engine, even though as I mentioned above there are at least 3 sets with that engine although 2 are mechanical. (late entry) I did find a mechanical "combination" set from the 1939 Wards catalog that included an 897 engine (note the 897 and 898 were numbers often used interchangeably on the same basic loco) . This set included 2 Bogota type, red/cream Pullmans and a red/cream observation and ALSO included some freight cars (and a beautiful lithographed station).
Just for your reference, the 1955 Sears Christmas Book, which does show your set (assuming the cars are red/cream) has for its number (i.e. Sears number)...49N U9604 "Passenger Set. Shpg wt 8 lbs"
According to the text in the Sears book... "It came with 2 sections of straight and 8 sections of curved track to make an oval of about 129 around". It came with a "UL approved 50-watt transformer with circuit breaker, 2 posts for 110-120 volt, 60 cycle AC and a track connector".
So there you have it, I admit for the moment I am out of reference sources that might help. We have a number but no set and a beautiful set with no number or ???? One thing about Marx and all these reference books say it more or less, he was CONSTANTLY changing, substituting, improving, altering etc. Lionel did not get to this stage until the 1960's after fair trade laws were repealed. Marx did it all the time. If you were a big buyer, tell Mr. Marx what you wanted in your sets (or more often the price point you were looking to fill) and Marx invented a set for you. If you wanted a "8-piece" or a "10 piece" or a "30 piece" set , Marx would fill up the box with whatever it took to bring the count up to where you wanted it. I have original sets that clearly had 6 telephone poles thrown in at what appears to be the last minute to raise the count level. He was producing toys not collectables and he was a genus at it.
Best of Luck with 8994, I have it entered the number into my mental data base and will continue to try and find information that might help
Don McErlean