I totally except I might be talking out of my bottom on this one, and 'real' world and 'theoretical' world don't always match up...but, here goes anyway...
The 'big' train is a creation of rail operators to reduce their crewing, line capacity and pathing costs. The 'big' train suits rail operators and it works for customers who just need their shipments to 'get there'. But where a 'big' train saves in 'on line' costs it loses money in terminal and utilisation costs. The 'big' train doesn't suit customers who want their shipments to get moving ASAP - that's why they use trucks.
In other words, for intermodal/general freight, 'big' trains stymie growth and ultimately cost shareholders their dividend. In the US - since at least the late forties - the most profitable railroads weren't the ones who tied every diesel to the front of the one daily train like SP, it was the ones who focussed on timely, frequent services - ATSF and the NKP.
Now this is where I'll talk a little crazy on terminal costs and utilisation - theoretically a 1500m train will take up the same amount of room and time to unload and load as two 750m trains. But the advantage of those two 750m trains is that one can be leaving when the other is arriving - which means those wagons and locomotives on a 750m wagon train spend only half the time sitting around in a terminal and are moving on the mainline instead. This is where utilisation savings kick in - over the course of a week/month/year the locomotives and wagons on the shorter train will make more return trips than one 1500m train, so theoretically over a year an operator might actually need fewer wagons and locomotives to run two 750m trains than running one 1500m train.
Now I know we can't go cutting an NR in half...but the 93/6000/LDP offer a game changer for the 'short' train theory that I don't think rail operators have really thought about. If two 93s can haul a 1500m train instead of three NRs, then a 750m train should be within the capacity of a single AC loco.
So in the end, the 'short' train costs more in pathing and crewing, but it saves money by reducing terminal dwell times and increasing asset utilisation. Whether that balances each other out, I don't know, but I'm pretty sure it creates a more attractive service for customers using trucks instead of trains at the moment.