The M-1 carbody is mostly conventional, well, conventional for Budd hehehehe.
There wasn't anything magic about it - Budd knew what they were doing, and they tended to be VERY good at it.
Besides, the M-1 sounds impressive until you see the specs on the Pioneer III cars - a full 11kv AC powered railcar that was *lighter* than the M-1s, and met AAR / ICC standards. The kicker was a really cool right angle drive and 6500rpm traction motors (!). The right angle drive didn't work too well though...
Then there were the origional unpowered pioneer cars for the PRR - full length, AAR / ICC (this was the 50's) compliant intercity car - 52,000 lbs.
Budd's trick was a decently strong stainless alloy (they claimed 50% reduction over carbon steel from that alone), a good design, and their 'shotweld' process, which yielded very strong welds. And they had a few good engineers (I have a 60's vintage paper presented by one where they talk about body hung TMs, AC inverters, and high speed DMUs as thing comming in the future. This was in the *60's*.
It's a shame they got out of the business. IIRC, it was BBD undercutting them on the R-62s for NYC that more or less popped them out for good...