Weathering H-Vans - What is this?

[Noel]

Watching one of the old 1970s YouTube clips of loose coupled goods train operations I noticed this staining effect on some of the H-Vans.

Question: What caused this distinctive staining/weathering effect? Splash back of buffer grease from buffer impacts, or water splash from the wheels of an adjacent wagon during wet weather (ie rain water from the rails thrown centrifugally from wheels like a bicycle without a mud guard)? The staining seems offset from the RHS buffer.

Edited March 31, 2019 by Noel
typo

[Mayner]

Common on the majority of  CIE 1950s 10' wheel base built freight stock, possibly more noticeable on wagons in the newer light grey compared to the old dark grey paint finished used up to the mid-late 1950s

Most likely track dirt (mainly steel dust and mud) thrown up by the wheels in  damp/wet weather.

 

[jhb171achill]

Yes, brake dust thrown up on the ends of the wagon.

Given the lighter shade of grey used on H vans & “palvans”, it was even more apparent. But four wheeled railway vehicles were more likely to show it up.

[DiveController]

Can you post a link to that video, would be interested to see... ?