Dealing with a double ended, two carriage train with nuclear fuel transferring plates, 12 seconds is un/loading and 3 seconds is the next train pulling up so (12/15)*54 is about 43 blue belts!

Of course trains will always take time to come in so I guess you could calculate the output ceiling by working out the ratio of time there's a train unloading to time the trains are coming and going

Based on a stack inserter transferring 27 items a second box to box, and 12 items a second onto an express belt, if you had six bottomless train carriages to unload you could support 54 almost compressed blue belts (27 items box to box * 72 provider chests = 1944 items a second, divide by 36 items a second from three stack inserters equals 54 belts carrying 36 a second)

Nice and neat. Good to have head room, it'll pay later. You can fill at least 16 blue belts with 3 lanes and two carriages but you'll need more ports and requesters