I must admit I default to 7 tiles far too much. I use all sorts of NewGRFs, but I am usually lazy and stick to 7 tiles. Usually I don't need to double head.
Experimenting with 14 tiles on a larger map at the moment though; I think it can work well for far flung industries as longer trains should mean fewer overall trains needed to maintain station ratings.
well i done the experiment
and in one of my route i broke the train into 3.5 trains
and then i run the do different scenarios for years and show the profit they made and they were the same
doubling the trains in the network i was afraid of jams but because of the short length everything was smooth and i can say it even work a bit better than 7 tile train
well it helped the special design of the station to load that permit multiple option routes for train and smaller train even broken blocked smaller part and depending where they broke some times they did not affect at all the traffic !!!!!!
while bigger trains if you play with breakdowns you will cost you a lot if break in the station blocking it
the experiment was to see if i can make more money due to faster deliver times but the industry i had was coal so there was not anything to notice.
also smaller train will pass faster junctions, ill need smaller turning space and many other goodies.
can you report you finding if you do the test and try 14 tile length train?/
can you have the game saved with 7 tile train and 14 tile train at same time more or less you will need some time to make the change and normalize the network
and tell us the profit was up or down??
are you planning to build 14 long length stations also??