Complex sentence: At 144 tons, the PR-8 was the heaviest conventional 4-8-0 ever built; the N&W M2, often credited as being the heaviest conventional 4-8-0, weighed 8,470 lb less than the PR-8.
Split into simpler sentences: At 144 tons, the PR-8 was the heaviest conventional 4-8-0 ever built. The Norfolk & Western's M2, which is often credited as being the heaviest conventional 4-8-0, weighed less than the PR-8.