In practice, that doesn't work out a lot of the time. The maximum permitted weight is the same for 20' and 40' containers (it's a function of the corner casting strength) but the 20' container is lighter. So normally heavy loads go in 20' containers, bulky loads in 40' ones. To get two 20' containers on one HGV they have to be loaded to less than half of their weight capacity - so usually either empties, or two loads of low density goods from different origins and/or destinations going in the same direction.The average 20' container is light enough that a HGV can load two of them, and you can distribute your containers across lorries to ensure they remain within tare weight restrictions.
The maximum permitted load on a container flat will allow for two fully loaded containers, either two 20' or a 20' and a 40'.
If nobody thought they were a good idea, then nobody would be willing to pay for them to run. If somebody was willing to pay more to run passsenger trains than to run freight trains, then the freight trains wouldn't be running.It just means someone is willing to pay for the trains to run, not that they are a good idea.
As far as train length goes, that longer trains are desirable isn't a surprise to anyone, and the industry has been working towards it. 775m trains are the current infrastructure planning baseline, and there's an aspiration for 1,500m trains.