Passport control on trains?

[JayH]

I agree - I've answered this before as well, as I have dual citizenship (New Zealand and British).  I used my New Zealand passport everywhere on my trip (as it had my Russian visa) except for my entry to and exit from Ukraine at Boryspil airport - there I used my British one, and it still has only those two stamps.

I read this some time ago and intended to comment.
A few years ago(3) a friend in Ukraine  asked me for help--a girl at her agency was expecting the arrival of  Australian guy at  DNE. She had in fact taken a taxi a few hours to meet guy who was in transit via Vienna.She had travelled at her own expense etc and paid deposit on apartment for him.She was at the airport waiting to meet flight and he was not on it.She called agency--agency owner is a friend to my friend(phew) and knew of relationship to me.She called me-- subsequently I got all the guys contact details-- and I called him to try and clarify situation.Ok--now to the point.
He was refused entry to the flight--based on his needing a visa as an Australian resident and passport holder.He tried to say he was going to use his UK passport to enter Ukraine--- this was refused as acceptable as it was not his current residence and regardless of UK passport he needed a visa as Australian resident.
Subsequently-- that position was confirmed.
I called his friend at airport waiting to meet him to explain to her--she was incredulous!!
I mention it here-- it does show how careful you need to be when using 2 passports and presuming that it will not be noticed--or that you can explain it away.
Last word--- I have been delayed boarding a flight to Ukraine and answering many questions--despite having visa in passport.