Mike:
" Most of LGB's engines had cars that came from the same railroad as the engine".
I have to disagree with your statement in the original post. In the beginning LGB made American style Narrow Gauge freight cars without an American engine. They ultimately came out with an Americanized Stanz with tender. Their first "American" style engine was the Red and Green Mogul lettered for the Denver, South Park, and Pacific. I am not aware of any DS&SP cars that LGB produced at the time they introduced that engine. Their next Mogul was lettered for the C&S. There were some C&S freight cars and a caboose, but no passenger cars. The SEG Mallet is no exception. Unless I am mistaken, they produced passenger cars for the three RR on the Glacier Express, but only engines for the RhB. There is the FO cog engine, but that wasn't used on the "Express". In the beginning they came out with many freight cars lettered for American RRs (Santa Fe, Union Pacific, Etc.). It wasn't until years later that they made diesels for those roads, but the eras and scale (gauge) were different.
LGB played fast and loose with railroads, scale, and gauge. They made great "toy" trains. They run and run.
The wheel base on the Mallet motor block is about 4" and on the Mogul it is about 5". Both motor blocks under the Mallet pivot. The shorter wheel base on the mallet is why it handles the R1 curves and the Mogul doesn't.
I have some questions that need answers before I could recommend a mogul over the mallet ( I have both, so I have no ax to grind either way):
1. Are you interested in North American Narrow Gauge or European?
The Mogul is American and the Mallet is German.
If you are leaning towards American, the LGB Uintah Mallet is an excellent choice.
2. What radius do you have on your layout?
If you have R1 curves on your track, over the long run you will be replacing Mogul idler gears on a regular basis. While they will traverse the 4' diameter curves they are not happy campers.
Chuck
PS I have been buying and enjoying LGB and other "G" gauge trains since 1980.