Modeling 1935-1945 can mean building models of military equipment as the Second World War dominated this period. Practically all military vehicles were transported by rail from factories or by units moving to other locations including to ports for overseas service. Interesting flat car loads of tanks and other military vehicles distinctively define this era.

At more than 49,000 made (including variants), the M4-series Sherman Tank was the most common U.S. made tank in WWII. It was also widely used by Allied armies like Great Britain, Canada, Australia, New Zealand, France, Poland, and the Soviet Union and some others.
Initially, they were made by eleven different manufacturers before production was rationalized into three plants later on. Railroad suppliers figured prominently: American Locomotive Company, Baldwin Locomotive Works, Lima Locomotive Works, Montreal Locomotive Works (Canada), Pacific Car & Foundry, Pressed Steel Car, and Pullman-Standard Car Manufacturing.
The Sherman tank came in many versions and its history is fascinating. The best place to discover and learn about the M4 is at the Sherman Minutia website (http://the.shadock.free.fr/sherman_minutia/index.html). All versions are covered logically along with identifying features. Predecessor designs are included. Comprehensive historical text is provided with many photographs.
Having this info allows one to correctly model these prototypes.
If you like holding a book in your hands, like me, these two are considered to be the best on the subject:

SHERMAN: A HISTORY OF THE AMERICAN MEDIUM TANK by R P Hunnicutt (Presidio Press, 1994) – note originally published in 1978. Republished in the 2000’s – these later printings do not have nice photograph reproduction due to paper used. This volume can still be found with book resellers like Biblio (http://biblio.com). For decades this was, and still is, considered the Sherman bible. A thorough history of all tank variants along with predecessor models & experimental types, with all subsequent designs built on this chassis (tank recovery, self-propelled artillery, tank destroyers, engineering specials, etc.). Extensive specifications and line drawings (1:48 O-scale) provided. Highly recommended.
THE SHERMAN DESIGN AND DEVELOPMENT by Patrick Stansell and Kurt Laughlin (Ampersand Group, 2013) – also known as SON OF SHERMAN Volume 1. Extensive information on identifying the small but significant differences between Sherman gun tanks only (no derivatives). A nice compliment to the Sherman Minutia website. Many photos and drawings, history of builders, serial and registration numbers. Highly recommended but hard to find (expensive).
Many other books on the Sherman are also available.
There have been many models of the Sherman offered in HO scale over the years. Unfortunately, nearly all of them are incorrect. We strive to make our railroad models as prototypically correct as possible, the same should also apply to the loads no matter what they are.
The best Shermans are offered by Artitec (https://www.artitecshop.com/en/military/h0-187/). They come as cast resin kits or very realistically built-up models, but it seems that the built-up models are more popular. As of writing the only Shermans available are built up.
A built-up model, as fine as it is, is also much more expensive than a kit. However, they do offer these properly configured as train loads (see choices on their website). In this category, their small hatch M4A1(75mm) of 1943-44 is quite good while their M4A3E8 of late 1944-45 can easily be adapted for transport.
Many of the built-ups are represented as tanks in the field, with interesting stowage and detailed accessories. Strictly speaking though, this would not be so for military vehicles being transported by rail in America. From the factory, or even a unit being moved, any loose items and tools would be stored inside the tank lest they go missing on route.
Alternatively, Alsacast offers 1:87 military vehicles (https://www.alsacast.nl/en/military-h0/tanks-vehicles/). These are only cast resin kits. The most accurate Sherman model they offer is the Sherman Vc (M4A4) Firefly, but these would not be seen on American rails as they were British conversions, up-gunned with 17-pounder. Unfortunately, they do not offer a straight M4A4 with 75mm gun. Their M4 and M4A2 models are incorrect as the bogie spacing is from the longer hulled M4A4. However, using these kits interesting kit-bashing opportunities exist. The cost is relatively low as well. Small Scale Hobbies (https://www.smallscalehobbies.com) imports these kits from the Netherlands.
In the future, I plan on posting how to convert an Alsacast Sherman into an accurate factory model that could be found on a flat car on U.S. rails.