Just to add some more to the picture:
The series 174000-174999 is listed in the 1953, 1955, 1959, and 1963
Equipment Registers with 50,000 lbs capacity! So as Richard said, even
if the cars occasionally went offline (a fact) the cars were clearly in
very restricted service.
The entire series is almost fully populated (nearly 1,000 cars) in the
1953, 1955, and 1959 ORER's -- but drops to 7 cars in 1963!! So clearly
the NYC either got rid of the cars between 1959 and 1963, or reassigned
them to another series.
However, the series 175025-175999 which I think were also Lot 737-B are
all listed with 110,000 lbs capacity. And this series remained populated
in the 1963 and 1965 ORER's. It makes me think that the "Pacemaker" cars
174000-174999 were somehow quite different. Was it just the trucks, or
some other factor?
OK. (Thanks, Ben!) So presuming the caption's accurate, it follows that these cars indeed wandered off-line, in violation of the rules, at least occasionally. To judge whether it was very rare or less so, I'd need to study a big stack of wheel reports or conductor's books. Gut feeling: modelling a Seaboard branch in Florida in 1949, I don't want one of these. I like my weird exceptions to be subtle enough that operators don't notice them.
Al Brown, Melbourne, Fla.