And did it while sprinting faster than a locomotive. He's just on a completely different level.
You'd probably lose that bet. From the interior/exterior of the Phantom Train, it looks like those are modeled after the Pullman type coach, a type of railway dining/sleeper/etc car used from the mid 1800s to mid 1900s.
en.wikipedia.org
A typical pullman car was roughly 85 feet long. There are 10 cars there, plus the engine. Assume they're all average length, that's about 935 feet, or about the length of a 93 story building if you stood it end to end. If you're from Philadelphia like I am, this is about the height of the Liberty Place or Comcast Center skyscrapers. Assuming Cloud/Tifa/Barrett are all 6 feet tall, you would need to stack 155 of them on top of each other to equal the length/height of the Phantom Train.
Each car is also wide enough that the party can move around freely within it as they would in a fairly large room which is WAY bigger than a pullman car. This is a BIG freaking train.
Diamond Weapon is nowhere near that size. Judging by the height of the party fighting it, it's 100-150 feet tall at best. Diamond Weapon also appears to be made at least somewhat of organic material- the train is not.
But wait, we're not done. The Phantom Train is SPEEDING at the time Sabin picks it up and suplexes it. To pull that off you need to overcome not only the weight of the train, but it's momentum. Or to put it simply, stopping the momentum of a 935 foot train that is speeding at you at 100 mph is a LOT harder than picking up one that's simply standing there- which is what Diamond Weapon is doing.
Any way you slice it, picking up the train is the more difficult feat.