The point is not about whether the language they seemed to be speaking is a potentially useful piece of information to give the police if you make the call.
The point is about whether their supposed nationality is in fact relevant to determining the likelihood that they are up to no good.
Being suspicious about people based on their nationality/appearance/whatever has consequences for people of that nationality/appearance whatever. Not theoretical consequences - real consequences. If there is an air of suspicion about "eastern Europeans" because people hear rumours that there are loads of eastern Europeans perpetrating cable thefts, this affects the way eastern Europeans are treated in their day-to-day lives.
So, each time this rumour is mentioned, say by the OP of this thread, it is reinforced. Hence why I feel it is important that it is questioned.
I suggest you stop trying to infer "Racism" where there clearly is none intended.
It is a fact that there are a considerable number of Eastern European criminals in the UK, many from Romania for example, were prisoners who were released by the Government and effectively deported to the UK.
There is a serious and growing issue with criminal gangs made up from Eastern Europeans. This is a fact and stating this does not constitute "Racism" in any way whatsoever. What I find deeply worrying is your rush to play the race card.
Whilst there are certainly a good percentage of Eastern Europeans who contribute to the UK, it is a fact that there is still a large group who are engaged in Criminality on a serious scale.
The OP was quite right to have been suspicious about people on and about the line who were seemingly more interested in the house alongside rather than the actual railway. I would have expected the OP to have reported boviously English speaking people who were acting in the same manner.
Oh and before you try to play the race card against me, I spend most of my time working outside the UK, where I am the foreigner.
I also speak at least one foreign language.