The Marx 1998 is a copy of the Alco S-3 Switcher. The Marx Rock Island power & dummy units (below) were the pair that turned my opinion of Marx from cheap and undesireable ,to an exciting toy train to collect and operate. This set was found at a hobby shop at which I had stopped on my way home, strictly by chance:

The 1998 series is cleverly engineered. Take a look at the underside of the next one you see. Note the method of power. Pretty clever! Also note below that the maroon Santa Fe below does not have  rubber "traction tires" indicating 1955 / early 1956 production. Note the horn is intact, and the diesel roar indicated by the silver screw underneath the horn, which is not on all units. The 1955 Santa Fe does not have mounting holes for diesel roar. Diesel roar was available on 1956 maroon or black Santa Fe switchers only:

All models from mid-1956 through the end of production, had 2 traction tires that made them decent pullers.

K-Line must have thought highly of this Marx model and when they purchased a group of Marx tooling the S-3 loco was one of the items. Though the powering mechanism is quite different the plastic shell bears a striking resemblance. K-Line produced the daylights our of this shell in many roadnames. An example of the K-Line offering is below:

The 1998 switchers had a relatively short period of  production from 1955 through 1962. The first roadnames offered were the Santa Fe, in maroon and black, plus the Union Pacific mostly in a  yellowish tan. Ever so few were made in lemon yellow which makes them very collectible I might add:

Of course, production dates are just that. Once inventory was sold from the factory it could remain in dealer inventories for many years. And many Marx items did!

The switchers were available in (powered form) with a working headlight. The maroon Santa Fe is the most commonly available as well as the least expensive. During 1956 production a Santa Fe maroon dummy was produced as well. I have only seen one unit like this and it was in an original box. It is basically the same as the powered unit but without the electric motor. The gears remain in the dummy but that is it. There are geared wheels on one side and non geared wheels on the other and no other gears. It has the diesel roar holes and the BLACK MOTOR FRAME; motor frames were changed to aluminum in 1957. There must be more around but not many. This is not a maroon unit but the undercarriage of the dummy is the same configuration:

In 1956 the Black Santa Fe switcher was introduced. Everything is the same as with the 1956 maroon version except there is not a dummy unit. It is pricier then the maroon unit but well within reach of most.

Also introduced, early in 1956 was the Union Pacific 1998. The innards were the same as the Maroon & Black Santa Fe's. Production of the Union Pacific, in 1956 only, came with a dummy unit:

Not too outrageously priced. The color introduced was the regular production yellowish-tan in the powered and dummy. It also came with "diesel roar" on some units which was the Marx version of diesel sounds.

Also issued was a distinct color variation, lemon yellow. Hard to describe but distinctly a different shade then the yellowish-tan. Almost translucent in appearance.This was only offered in a powered unit and for a very short period of time and is very difficult to find. Probably a production variation caused by a different shade of yellow plastic then normally used:

The next unit introduced was in 1959. This was the blue AllState unit. It only came as a powered unit and is reasonably tough to find, but, certainly not rare:

There were at least 2 distinct colors produced as original. Do not store it in the sunlight as it will fade and turn into additional shades of blue!

Also introduced in 1962 was the Rock Island. The body was red and gray and is the first picture in this article. It was only offered in powered and dummy unit conbinations with silver trucks and black or white lettering.

On an overall basis, the Marx Alco switchers are quite sturdy. Make sure that the rear overhang is not chipped or missing a piece if "factory like" is your aim.

This wonderful collectible and operating series are fun to find and further add to the lure of Marx trains.

Very best, Mike (lionelsuperotrack on eBay)

PS I would like to acknowledge John Fox. & Tasker Brush for introducing me to Marx trains. Their continued willingness to answer my questions and share information is most appreciated. Their input has been invaluable and gracious in every instance.