As discussed in #635 and #1461 , there needs to be a cart that locomotives can draw both liquids and fuel from. A "Tender Cart", if you will.

The Tender Cart would hold fuels and water (Not sure if the temperature of water from installs with BC's new liquid temp feature should be noted), but in a more limited capacity then a tank cart plus a cargo/chest cart would, but allow for the locomotive to go farther without manually refueling/watering a locomotive, especially useful on long distance trips or constructing longer railway segments.

Edit: if playing with BC also installed, the hotter the water, the less efficient tank/tender carts should be at keeping the locomotive cool.

Edit 2: The Tender cart would look like a cargo cart, but be filled halfway by a half-sized tank tipped on it's side towards the front, and room for the fuel in back.

Not a good first issue: This needs more discussion on how exactly this fits here.

So after some discussion on the discord, we came up with this being the better of two options:
hat if you could use the tender cart for each locomotive, but depending on the locomotive it was attached to, it could haul different things, in addition to sand to help with traction?
Electric locomotive tenders would store extra charge, allowing them to perform better at lower or nonexistant voltage levels until the stored extra charge ran out
Diesel locomotive tenders would store extra diesel fuel
Steam locomotive tenders would water + fuel
Tank locomotive tenders would store steam (The tank locomotive's version of fuel) and be able to store the water from when the steam finally cools down into water
And the thaumcraft locomotive would store thaumcraft's magic resource thingy (Was it mana or magika? I can't remember)

Maybe you can also do some interesting mixing of Tender-Carts. You can update your Steam tender to a coal dust tender and insert a coal dust variant of a steam locomotive. Maybe it has the same consumption rate of a coal dust to coal equivalent, but slightly more horse power.
Another combination could be a steam / oil Tender cart for a oil fired Steamer.

Currently i run on my personel server a 1.7.10 world with big biomes. My track net has round about 35kBlocks length crossing desserts and so on. A Tender cart would be a very handy thing in such worlds.

I thought about writing covert about this, a while ago but figured no one would go for it. Guess I was wrong! Here’s my idea. I think of railcraft as more a miniature railroad transportation system. And in miniature railroads typically… the engineer sits on the tender and operates the locomotive. I think the tender should be a source for coal that feeds directly to the steam locomotive and has seat so you can control the locomotive as you travel down the rails…or just let it run on its own. Now to fill the tender there should be a coaling tower, much like the water tower feeds the locomotive, the coaling tower feeds the tender. Only things is, you have to fill the coaling tower with coal. You can place coaling towers and water towers all over your world… To bring coal to the towers put a hopper in a mine cart making a hopper cart and fill up a bunch of them with coal. Send the train to your towers, pipe the coal into your towers… the coaling tower should also be a multiblock structure 3x2x5 think railcraft is really missing out on this…
Another idea for the water tender is. If you look at a garret AC30 steam locomotive it has a water tender in front of it and a coal tender in back. Another possibility for those who want to run with more water.
If I could code I would have made this already….

Possible Tenders to include:
Front Water Tender
Coal + Seat Tender
Coal + Water + Seat Tender
Oil + water + Seat Tender (for when BC is installed)
Coaling Tower

? When is the 1.12.s version supposed to be ready. I really miss it.