The train only lays curved tracks; if I try to lay straight tracks, I get an error message (see image):
https://steamcommunity.com/sharedfiles/filedetails/?id=3687844473
As you can see in the image, I have everything I need in the car.

You are using a mod where the ghost rail is not a vanilla rail (drp-mockplanner-rail) and you have vanilla rails in the train inventory. That's why it cannot place them

But I used the standard tracks... And he’s laying the curves too. Aren’t they exactly the same tracks?
The construction robots lay the tracks without any problems either. I don’t have any other tracks in the construction menu, not even when I display everything regardless of research.
I don’t understand that.

EDIT:
I found the culprit... It was the Dual Rail Planner.
When I disable it, it works again. Too bad, the Dual Planner is really cool.

I will still look if there is something I can do to circumvent this problem, but the mod cannot strictly place a rail variant that it don't have in stock (if you have space rails from Space Exploration, my mod will check if you have space rails to place them)