That's true, but I like to get my conquesting underway as soon as I have chivalry (for Japanese or Arabs) or Military Tradition (for Ottomans and others). If I waited to have my railroad network up, the enemy would have musketmen in all their cities and possibly even cavalry. By the time I got to later civs, they'd have riflemen and that just reeks of a fair fight.
Oh yes, and if you like Ancient wonders, do not stack the game with the new civs, because they all have early UUs and thus early golden ages. I just got Conquests and wanted to meet the new guys, and ended up only getting Great Library and Temple of Artemis, not pyramids, and that was on Warlord level!