Here’s a question from the search engines: Can you mix Lionel and Marx track?
Generally speaking, yes you can. Just stick with O27 track, and you can mix Lionel, Marx, and K-Line as needed. Dad had a mixture of Lionel and Marx track in the 1950s–my theory is that someone tipped my grandfather off that you could buy a Lionel O27 starter set, expand it with cheaper Marx track, and once you had the track assembled, no one would know the difference. When we set his layout back up in the mid 1980s, we added some K-Line O27 track, because it was what we could find. I have a mixture of all three brands to this day.
The easiest way to identify O27 track is to go by the track tie color. Since 1970, Lionel O27 track has had brown ties, and the larger O31 track has black ties. K-Line adopted the same practice. If in doubt, buy track with the brown ties and you’ll be OK. All Marx track was the smaller O27 profile.
Some people mix O27 and O31 track, but you have to resort to shimming the track and either crimping down the O31 track to accept the smaller O27 track pins, or enlarging the O27 track to accept the O31 track pins. Even then, the heavier gauge of the O31 track causes a bit of a bump at the transition, so I don’t recommend the practice.
There are some benefits to mixing track. Marx made curves 27 and 34 inches in diameter. K-Line made it in 42, 54, and 72-inch diameter, which essentially forced Lionel to answer with 42 and 54-inch diameter itself, though Lionel unfortunately never made any 72-inch track in O27. K-Line O72 track isn’t easy to find today, as it’s been out of production for nearly a decade now. Trains run smoother and derail less on the wider turns, so my layout is almost all 34, 42 and 54-inch curves now.
In Ebay listings, you can identify Marx’s wider curves by the number of ties. O27 track has three ties. Marx O34 track has five. Unfortunately you have to eyeball the wider-radius Lionel and K-Line track if the seller doesn’t state the diameter outright.