The idea is that the so-called death camps were really transit camps, and that they were located were the gauge changed: Russia had a different gauge from Germany and Poland. Thus, since the trains had to be changed, it was logical from the Nazi's point of view to bring deportees first there, maybe shave and disinfest them and then send them further east.
This is a map of Poland in 1918:
You can see the river Bug along the east border of the blue area and then bend inwards where "PO" of "Polska" is written.
Compare the location of the alleged death camps, Sobibor and Treblinka close to the Bug river and Belzec some kilometres off:

So somewhere near Sobibor the gauge must have changed. Check also the position of Belzec between Lublin and Lvov on the second map to see that it was just on the border of Poland in 1918 as shown on the first map. But what about Treblinka?
Treblinka isn't on the border. So why would the gauge change there? This is resolved by viewing the German-Soviet demarcation line after the occupation of Poland:

Thus, Russia had extended its territory along the Bug river, just to where Treblinka (or more precisely, Malkinia) is situated (at the point where the Bug river turns south; the third map looks a little distorted probably due to a different projection). If we assume Russia had changed the gauge to its own by 1942 or so, then all the main "death camps" were built exactly where the gauge changed. What a coincidence!
Today, Belzec and Sobibor are still located on the border.
http://www.skiba.nl/Poland/Poland_map1.gif
And people are still taking a break where the gauge changes: Bialystok, also situated on the border, is the main train stop on routes from Warsaw to Minsk/Riga/Wilna according to German Wikipedia.