These are the rules effecting the units they are referring to.
19.5 The Nazi-Soviet pact
In all World in Flames scenarios that start after Jul/Aug 1939 and before Jul/Aug 1941, the USSR and Germany have a Nazi-Soviet pact in place that regulates their spheres of influence. They signed their pact on the 23rd August 1939 and Germany broke certain of its clauses on the 22nd of June 1941.
19.5.1 Eastern Poland
The USSR can exercise its Nazi-Soviet Pact rights to occupy eastern Poland during any Allied land movement step. However, it can only exercise those rights if Poland has not been conquered. You exercise those rights by moving a land unit into any hex of eastern Poland.
Once you exercise those rights, the part of Poland to the east of the partition line becomes conquered by the Soviets. Move any Axis units there to the nearest Axis controlled hex they can stack in. Any Allied (except Soviet) units there are destroyed. They are removed from the game (internment) until Germany and the USSR are at war, at which point they may be added to the Commonwealth force pool if the Commonwealth player so desires.
Railway supply paths
A hex a railway supply path enters, by moving along a railway or road, does not count against the 4 hex limit. A hex it enters across a straits hexside also does not count against the limit, so long as the hexes on either side of the straits are railway hexes.
The 4 non-rail hexes can occur anywhere along the path. Although you will mostly use them to trace supply from an HQ to the railhead, they can be handy for re-routing around an enemy unit that’s blocking a vital rail link.
Perhaps they can explain why the rail hexes in Poland cannot be used to supply the units in Brest-Litovsk.
Because it is not a railway supply path. Railway supply paths are for 2ndS relaying, normal units always use a basic supply path.
“WiF is like sex: sure, it may give some practical results, but that's not why we do it.”
- Richard P. Feynman, 'WiF, Sex, and the Dual Slit Experiment'.