This kata was published in 2013. Ranks have deflated since then, and they cannot be changed.

I totally agree. This was much too easy for a 4 kyu.

What you're meant to assume (it's not stated explicitly) is that ...

Actually, it is stated:

• A train that looks like zzzzzZ is travelling clockwise as it passed the track "zero position"
• A train that looks like Zzzzzz is traveliing anti-clockwise as it passes the track "zero position"

Kata already says:

The engine and carriages use the same character, but because the only engine is uppercase you can tell which way the train is going.

and

All tracks are single continuous loops

That's all you need to know.

ALL trains go "forwards" (with the engine at the front!). I thought that was so obvious I did not ever write it ;-)

Of course, just looking at aaaaaA on a complex track, it will be difficult to judge is that train really going "clockwise" or "anti-clockwise" as it passes the track "zero position".

It's not the author who gives the kyu.

Well, then your tests should be catching them ;-)