Originally Posted by hsmall

The other reason (if I may be geeky for a minute and unless things have changed in the last decade or so) is that there is a break of rail gauge at Bialystok. Countries such as Finland, the Baltics and Belarus (Byelorussia if you prefer) which had the inestimable privilege of being ruled by the Russians in the 19th century used the Russian railway gauge of 5' rather than the standard gauge of 4' 8.5". So if there were through trains into Poland they would have to have the carriages jacked up and the standard gauge bogies rolled on in place of the Russian ones.