TOPIC:

It is possible to do the bidirectional transportation using different types of cargoes on FIRS server, i.e. transport one cargo in one direction and another cargo in opposite direction. This become possible due to automatic refitting of trains at stations, which can be specified in "Orders". For example, I can refit train to oil and load it at station A, and then unload oil and refit to chemicals and load them at station B, then unload chemical at station C. After that train can go to station A again forming a loop. The same behaviour player expects for other wagon types (not just tank wagon), but behaviour appears to be different. For example, if I load sand at station A, refit train to building materials at station B, then the train will never be refitted back to sand at stations.

that depends on the newgrf's used and the wagons picked.
Have you tried this?
Example on server #30-Temperate - Citybuilder:


Or this one, tested on server #9-RandomClimates - Company Value:


From what I saw, you need to select the station order to select the "refit" order changes.

I think those are the 2 servers we have using FIRS and the sets used allow to "refit on station". I don't to test it, but options are there so, I guess they should work ^^

Hope it helps

ST2 wrote: options are there so, I guess they should work ^^

Yes, options are there, but they do not work. Test it: try to refit train at station to building materials and then refit train to sand at other station. There is only bulk wagon that can be used for that. Flatbed wagon does not allow to transport sand.
The train will refit to sand and to building materials, but after refitting to building materials it won't refit to sand.
I have encountered that problem on server #9.

The table below shows all possible refits at stations for bulk wagon. Green means refit is possible. Red means refit is impossible.
Though refits marked with red can be done in depots, this leads to potential jams, when added to vehicle orders.