How to attach trailer?

[Limonene]

I have a bus and a trailer lined up perfectly, with the bus's trailer connector knob exactly coinciding the hole in the front of the trailer's connector. However, they do not attach, and there is no hint of how to attach them.

I have tried several combinations of wheel to get the bus and trailer to exactly the same height, with no change in behavior.

I would have expected either for them to attach immediately, or a hint to appear in the bottom left corner of the screen, telling which button I could press to attach them.

[Whett Faart]

You grab the hitch of the trailer in your hand, drag it over to the hitch on your vehicle. If you're close enough, you'll see a red thumb tack. Then you just hit the attach key.

But if you're playing on the test branch, trailers are broken.

[Limonene]

Thanks, I'm on the test branch, so it must be that. (The main branch is years out of date.)