Distance between turrets
It can often be an advantage not to build your turrets too close together at the beginning of a game. Else it might happen that two turrets fire simultaneously at the same (small) container, although a single shot would have sufficed for its destruction. This way the second turret would have fired for nothing, but it would still need to reload and it could only fire delayed at the next target - which could already have passed the turret at this time.
During the further course of the game, when larger containers are transported by the trains, this circumstance is no longer that important.

Positioning of turrets
Especially on maps with a linear route of the rails (without any loops), the positioning of turrets can be crucial for your success. Usually you should position your turrets in a way such that they will hit all or the most part of the trains that are still to come. At the same time you should build as close as possible to the start location of the rails. This way you will destroy the trains as soon as possible, you get the reward money earlier and you can build your next turrets even faster.

Curves
Curve speed
Because trains do slow down in front of curves and are also moving with reduced speed through them, sometimes it can be helpful if you build your turrets in the immediate vincinity of curves. This way these turrets will potentially have a better yield.
Especially at the beginning of a game this approach may provide you with a crucial advantage.

Pausing the game
Some situations in the game can be rather challenging because often you will have to react very quickly. To not lose the overview in such moments, you may pause the game which gives you the opportunity to analyze the current situation. If you are able to come up with a solution this way, you should try to implement it subsequently without pausing the game, so that you are able to reach the highest possible score. Because playing the game without pausing is more challenging, you will also get a higher score for it.

Level of difficulty
A low level of difficulty is often appropriate if you have problems and are stuck on a certain map. On the other hand the attainable score is directly dependent on the level of difficulty. So if your goal is to reach the highest score that is possible, you will only be able to do that on the highest level of difficulty.

Save credits
On maps that contain loops it can pay off if you only upgrade the turrets in the loop for the last group of trains to their maximum level. Because the other turrets will have more time to destroy the previous groups of trains, they will not necessarily need to be fully upgraded.

Divide the last group of trains
On maps with multiple loops it is sometimes possible to distribute the last group of trains into several loops. This way you can save important time which will in turn reward you with a higher score!

removed delay
Remove delay for a group of trains
If you are fast and you do not want to wait for the next group of trains, you can click on the corresponding group of trains on the right side of the screen to remove or shorten the delay. This way you can (if possible) save some time, which will give you a higher score.
Especially on the highest level of difficulty, it will not be possible for you to remove the complete delay for a group of trains. But in some situations you can save some small amounts of time here and there.