TL;DRLocomotives facing the opposite direction should contribute to the propulsion power of the train.
What ?Currently, a train in Factorio is only propulsed by all locomotives facing the direction the train is going. For example, if you have a bidirectional train with one locomotive on each end facing opposite directions, only one locomotive will pull the train while the other locomotive will be doing nothing, other than making the train slower due to its additional weight.
I am suggesting that all locomotives should contribute to the propulsion power of the train, even the ones that are facing the opposite direction.
Why ?In reality, a modern bidirectional train with a locomotive on both ends will normally be propulsed by both locomotives. One locomotive will pull the train and the other will push the train, so that the total propulsion power of the train will be the sum of the power of both locomotives.
Therefore, in reality, bidirectional trains with 2 locomotives have no disadvantage compared to a unidirectional train with an equal number of locomotives. In Factorio, however, bidirectional trains only have half the power of a unidirectional train with an equal number of locomotives, because, in Factorio, locomotives must be facing the "correct" direction in order to contribute to the propulsion power of the train. Due to this, bidirectional trains have a major disadvantage in Factorio, which they do not have in reality.
As a consequence of this "nerfing" of bidirectional trains in Factorio, they are not very popular. Players often build unidirectional trains instead, because that is the only way to get the full power out of all locomotives. However, using unidirectional trains also requires that the player builds track loops everywhere, which would not be required for bidirectional trains.
Personally, I do not like the need to build track loops everywhere, because, in reality, they are unnecessary, for the reasons described above. In reality, virtually all trains are bidirectional. Even trains with only one locomotive are effectively bidirectional in reality, because the locomotive would simply detach itself from one end of the train and attach itself to the other end, if the train must change direction. This is possible because modern locomotives are symmetrical, i.e. they can drive equally well in both directions.
Of course, a locomotive automatically detaching itself and reattaching itself to the other end of the train would be too much work to implement in Factorio. (EDIT: Meanwhile, a mod which allows exactly this has been created for Factorio.)Therefore, I am not proposing that Factorio should attempt to imitate reality in every aspect.
However, I do believe that, in Factorio, a bidirectional train with at least one locomotive on each end should have the full amount of power of all of its locomotives at its disposal, no matter in which direction it is driving.
Extension of the suggestionIf this suggestion were implemented, it would no longer make sense for locomotives to have a front and a rear, since they could drive in both directions equally well. In that respect, locomotives would be symmetrical. Therefore, it would also make sense to also change the graphics of locomotives to make them symmetrical.
This would also be realistic, since most modern locomotives are symmetrical.
EDIT: Updated my original post with the suggestion template.