here is the idea:
overhaul of the boiler:
now smaller boilers heat up faster, have more pressure, but less raw ammounts of steam, meaning they drive less pistons at higher speeds
bigger boilers are the contrary
you can attach fireboxes to the sides of a boiler to increase its temperature further
turbine now makes rotational power, being essentialy a more compact version of the pistons at the cost of not benefiting that much from scaling
whe also got the dynamo, a version of the generator that works essentialy as a gearbox, allowing you to squeeze more power out of a shaft at the cost of not being able to use this shaft for engine power
nuclear reactor:
the nuclear reactor consists of modules that are 4 blocks long but 1 block thick, they are fuel rods (generate flux and temperature when hit by flux) control rods (control the ammount of flux that gets through) reflectors (relfect flux back at the direction it came from) steam chanels (turn reactor heat into steam) and the starter charge (makes small ammounts of flux, just enough to cause the fuel rods to become self sustaining), if its temperature gets too high (namely 1500 degrees) it causes a explosion that scales with its size
i know its quite taboo to suggest a nuclear reactor, but cmon, this one is simple, and fairly realistic, also, devs, if yall want to add this, feel free to change the numbers, i am saying what i know from HBMNTM