The earlier the TRN network starts, the earlier Africa is retaken, the earlier Persia can be reinforced against attack from India, the sooner W. Europe and S. Europe can be threatened and exchanged with Germany, bleeding off units from the eastern front.
Running infantry from EUS to ECA is a necessary evil because you don’t want to be stuck with three transports per two transported ground units. But running infantry from WUS to WCA to ECA means that whatever’s in WCA are units that could have been at Germany one turn sooner.
If Germany doesn’t send the Med fleet west, I often unite UK battleship, 2 UK transports, 1 US destroyer, 2 US transports west of Algeria, with total UK forces 2 infantry 1 artillery 1 tank, US forces 2 infantry 1 artillery 1 tank 1 fighter at Algeria. My build on US1 of 3 transports, 3 tanks, and 1 infantry lets me hit again next turn. US2 I build 1 transport plus 8-10 ground (if Germany has run through Africa, I get tanks), and send the 2 empty transports from Algeria back to E. US, and send full transports to Algeria. US3 I send 3 full transports back, run 4 ground units to ECA, and put 3 empties back on EUS, build 8-10 more ground. US4 I use 2 empty Algeria transports to grab units from ECA and send 3 full transports from EUS to Algeria, so now there’s 10 units in Algeria threatening WEU and SEU with UK help - and more on the way from a feed from EUS-ECA. There is NO TIME TO SPARE; the US landing on US1 allows progression US1 Algeria, US2 Libya, US3 Anglo-Egypt, US4 Trans-Jordan. If the US does not press hard and fast through Africa, Japan secures Persia to threaten the Caucasus, almost impossible for the Allies to crack an infantry wall there, because the Allies must ATTACK to take that crucial territory. But if the Allies manage to unite at Persia on US5, Japan is probably NOT going to be able to push ground through the Caucasus; it will have to take on units from three different countries; US, UK, and USSR - not much from each, but the combined strength is very great.
That is why I don’t think I would ever run infantry through WUS. I would let Japan hit me, early or late. Early, and that’s less early pressure against Russia (good). Late, and I can deal with it. (good). (Note that I didn’t say that late, it’s less pressure on Russia - because late game, infantry transported to Soviet Far East - Yakut - Novosibirsk would take TOO LONG to get to Russia. It is far more effective for Japan to just open a can of whoop-ass with eight transports and start slugging Alaska and Canada. Problem is, by that time, UK should have its transport fleet set against Germany, and the US can use its already mobilized units near Europe, so it’s UK/USSR/US (in order of strength) hitting Germany, USSR vs Japan, and US playing a back seat role in keeping Japan from reinforcing its Asia position.