Complex sentence: They usually consisted of 12 carriages divided into three blocks of four coaches--one of these blocks would be a powerful 3200hp 4 REP electric multiple unit, while the leading blocks would be "trailing"--unpowered--4TC multiple units.
Split into simpler sentences: They usually consisted of twelve carriages divided into three blocks of four coaches each. The trailing block would be a powerful 3200hp 4 REP electric multiple unit, while the leading blocks would be unpowered 4TC multiple units.