Comment by lbriner

I feel like this is one of those articles that claims some clever idea on paper but without experience of how this would actually work IRL but IDK if the OP has more experience than me so I will assume the best.

Others have already pointed out some of the practical things that would limit what you can achieve with shorter trains like minimum headway between trains and the congestion that would occur after a delay where you would expect people to wait for maybe 10 short trains to arrive and depart before you could get on.

There are other problems. The cost of the smaller trains means if e.g. 2 carriages each, then every other car is a driving vehicle with motors and control equipment. On longer trains, this could be 1 in 3 or 1 in 4. Each of these not only requires regular maintenance (so that's doubled the maintenance requirements) but also creates massive congestion issues in maintenance yards. Commonly, 1 or 2 full trains fit in each siding so getting most trains out if e.g. one is broken down is usually easy enough. Imagine having to move 3 or 4 separate smaller trains out of the way, they are not automated in the yards.

Most people would be very unhappy knowing that they might be alone on a train, which is a main reason why operators are not quick to get rid of all staff. But currently, a single driver and conductor is close by for, maybe, 8 carriages. This would double if you needed a single attendent on each 2 carriage train.

The signalling is very heavily designed around traditional trains with its delay after passing a signal. 4 x 2 carriage trains would utilise more signalling capacity/time than 1 x 8 carriage train. If these were all coming from the same location and needed to return, that would also need a lot more platforms so that the following trains don't block the first trains in terminal platforms. This is already a major problem at most UK terminal stations so that is largely unsurmoutable.

I also heavily question the idea that station cost is largely dependent on platform length. On most surface stations, platforms are relatively easy to construct and whether they are 4 carriages or 12 carriages long still require a station building or 2, some ticket machines and CCTV etc. For elevated railways that is more likely to be true maybe.

So yeah, like someone else said, we are unlikely to come up with ideas that 1000 other engineers haven't already asked, the easier problems to solve are around planning, design, legal rights for infrastructure projects etc. since these tend to eat up sometimes decades and billions of dollars.