The 70 with added lead only weighed 450g, the 68 weighs in at 700g with no 'adjustment' required.
I didn't find a speed problem with the 70, it's not an express locomotive, it's a goods loco, just its FAR too light.
I was so disappointed I sold my 70.
I've one 68, and am after another - both TPE's. Superb locomotive.
Bachmann got the detailing very nicely on the 70, but that's if it stays on a shelf somewhere.
Nowhere near representative in 'prototypical running'.
Al.