With all due respect, but I come to other conclusions…

Tier 4 diesel locomotives are indeed progress. However, they are still internal combustion engines, which carry their power plant along.

In order to move those 10-car trains at the require accelerations, you will need a high power-to-weight ratio as well as high tractive force. High power-to-weight ratio can be achieved by adding more power or/and by reducing weight (whereas the latter is preferable). Diesel engines tend to be heavy, and the more power is installed, the heavier they get (and don't forget the additional weight added by the fuel). We should also not forget that locomotives add non-productive weight to the train (non-productive means that they are not carrying any passengers).

The conclusion from that is that motor units perform better than locomotive-hauled. Motor units can have distributed power, are therefore lighter, and do also carry passengers.

However, we still have the limiting factor of internal-combustion engines: The power output is limited to what the prime mover can provide. And only a part of that power goes to traction; the other part goes to auxiliary components, and Head End Power (aka air condition/heating, lighting etc. of the cars).

That leads to separating the power generation from the vehicle (in other words, electrification). In fact, the limiting factor of power output of an electric locomotive/motor unit is the thermal load on the armatures and power electronic components. That means that, providing sufficient cooling, it is possible to overload an electric unit for a short time (meaning acceleration). That is the pure traction power; additional consumers (auxiliary components, head end power etc.) is not affected.

We can consider the Caltrain line to have the potential of a high-performance line. High performance can, as shown above, only be achieved with high-performance rolling stock.

In German speaking countries, there is the saying "Organisation vor Elektronik vor Beton" when it comes to capacity and performance of a line. That means the first things to do is optimizing schedules and procedures, or increasing the frequency. When that is not sufficient, do things "electric/electronic", such as signalling, electrifying, powerful rolling stock (OK, some concrete or wood would be needed to provide level boarding, which is a combination of organization and rolling stock measures). Only when that is not sufficient, start using concrete.

Lengthening trains has its consequence that station platforms have to be adjusted. It is some time back that I rode Caltrain, but from discussions here and elsewhere, it can be difficult to achieve at some places. As desirable as it is, to provide a seat for everyone, it may be better to provide sufficient standing space for short-distance travellers. And there is that particular Caltrain issue, the bicyclists…

One approach to reduce overcrowding is to increase the frequency with powerful rolling stock, which may be shorter and have fewer seats, but overall there will be more seats offered.

All that said, Caltrain is, despite of all the shortcomings discussed here and elsewhere, on the right path. An with only a minuscule political will, California is able to finance the whole project, without having to beg for "federal money", which is hard to get with the current "regime".