Complex sentence: Over time, the RFU and RFL adopted different rules and the two forms of the game--rugby union and rugby league--diverged, and to this day rugby league's stronghold remains Northern England along the "M62 corridor".
Split into simpler sentences: Over time, the RFU and RFL adopted different rules and the two forms of the game--rugby union and rugby league--diverged. Rugby league's stronghold remains Northern England along the "M62 corridor".