From time to time I think about which features a new built modern steam loco should have to be financially and environmentally superior to current locomotives.

You are welcome to contribute your thoughts!

Let me start with some requirements:
- one man operated
- low fuel cost - which does not explicitely force a very high efficiency!
- low maintenance cost
- carbon neutrality in today's meaning
- quick starting up
- high speed capability

To be said first: Several of these requirements imho are solved by one partially new solution:
https://mackwell.co.nz/replacement-boilers/

- No more risk of explosion gives the opportunity of automatic boiler operation without any bureaucracy. This allows one man operation for the locomotive.

- Using waste bio fuel massively decreases the cost of fuel consumption and results in carbon neutrality, related to today's interpretation. The still rather low efficiency of the steam cycle does not undermine this.

- The boiler solution with water tubes can be steamed up a lot faster, in standard gauge perhaps within less than an hour, using the best solutions for the first phase after lightning.
In my opinion, this is still not perfect and can make new steam locos less competitive in some of their possible applications, but imho carbon neutrality is still far enough "saving" the comparison.

The current "Modern Steam" solutions from DLM in Switzerland and from many other rather modern applications ensure low maintenance cost and time consumption.

So after all this, the remaining point is high speed capability.
Today, the limiting factor for this is inertia, and at a first glance, the next limiting factor is the limitation of the sectional areas for steam flow.
But at a second glance, we see that also the se areas finally are mostly limited by inertia because larger valve piston travel would result in inacceptable amounts of inertia.
To solve this all, mass balancing has to be far extended. And the good news is that this is possible.

The ACE 3000 project with its 4-4-4-4 design, which was a 4-8-4 design concerning its tractive effort because of the connection between both engine sets, was a good first shot. Better balancing and/or less valve gear parts can be achieved on the same fixed wheel base like the PRR T1 but with smaller drivers, e.g. 72" instead of 80" diameter, and two engine sets with a common cylinder block in the middle of the loco.
This way, a good horizontal balancing of the main drives can be achieved, and a smaller amount of valve pistons or pairs of opposite directed and so balanced valve pistons (with crossed inside/outside admission) can be achieved. So the weight of the valve pistons does not need to be balanced any more even for high speed, and the pairs of valve pistons even can have double admission on each valve piston. So the sectional areas for steam flow can have up to four times the size (per cylinder) from before, based on the same diameter of the valve pistons.

For more background info, you are welcome to take a look at the concepts page on my steamtec website, https://steamtec.s-a-s.ch/?box=concepts ... 0balancing

Your comments are welcome!

Poppet valves may help offset the mass inertia of the piston valves.

Tubo charger driven off exhaust steam to blow on fire?

@RCD: A turbo charger will be less efficient than the regular means. Only in condensing locos there was no better option.

@Dave: I do not like poppet valves for steam locos at all, but yes, they could be the best solution.
But as I wrote, when piston valves can be balanced, things could be different.

Well obviously the most important feature a new built modern steam loco should have, is air conditioning!