Here are some base facts:
- can hold 75k units of a liquid.
- is heavier than cargo wagon. Train doesn't accelerate as fast as with cargo wagons.
- can be loaded/unloaded extremely fast. It can be filled in just 4 seconds. I wasn't able to measure exact unloading speed, but I assume it is same as the loading speed.
- can hold 100k units of a liquid.
- swapping full barrels for empty ones is also pretty quick, thanks to barrels' small stack size.
I made a test to see how fast a train with cargo wagons and a train with fluid wagons brings 900k water from a pumping station. I used trains with 3 wagons. I actually made two tests, one with the pumping station 100 rail pieces away and the other one with a distance of 500 rail pieces. I used 6 assembly machines 3 filled with Speed modules 3 to do the barreling job.
Here are the results:
- Long distance test: Fluid wagon train: 4 minutes, 26 seconds; Cargo wagon train: 3 minutes, 38 seconds
- Short distance test: Fluid wagon train: 2 minutes, 26 seconds; Cargo wagon train: 3 minutes, 15 seconds
- The actual target amount of water was 898k, because residue water in pumps cannot be read by circuit network. The idea was to have cargo wagon train make 3 trips and fluid wagon one 4 trips to account for the difference in capacity.
- Cargo wagon train didn't perform much better in short distance test, because it was waiting for previous batch of barrels to be filled. I used 3 sets of barrels. One waiting in each station and one being carried by the train.
The test made me aware that carrying capacity of a wagon is not the only comparing criterion. Another one is to compare how fast the wagons can feed their contents to the piping system. And while the cargo wagon carries more water faster, the fluid wagon unloads faster. 75k liquid in 4 seconds is 18750 liquid per second per wagon. 1 Assembly machine 3 with Speed modules 3 gives 937,5 liquid per second (Base recipe: 250 liquid per second and 3,75 crafting speed). To match the fluid wagon number, you'd need 20 moduled Assembly machines. And that doesn't count for moving the barrels. 40 assemblies would double the barreling speed, leaving you with 2 seconds to unload the barrels, split them and feed them to the assemblies. I don't think that is possible. Also, that amount is per wagon. With 3 wagons, you'd get gigantic unloading station.
That speed is not really needed, however. If the trip is long enough, the barreling process finishes while the train is going. Therefore I recommed fluid wagons for shorter trips and cargo wagons for longer ones.
I'm also uploading saves I used for testing. Just turn on the power switch in the middle (on screen when you load) and watch the trains go at it. The numbers there show how much water (in k) is already transported. There are also timers near destination stations, which stop when target amount of water is reached.
- Cargo wagon train seems to always sneak one more load of barrels before it is stopped, causing the 3 digit display to overflow.
- Factorio ticks 60 times per second. But when I had the timer show 60 ticks as a second I ended up with a really slow clock. When I tweaked the timer to treat 30 ticks as a second, I finally got it to show proper time. I am not sure why.