So now that the 20th CL is out...

[peteski]

Thanks guys - I learned something new.   

[davefoxx]

Generally, Pullman Standard stainless fluting was applied over the Corten steel sides- essentially all PS cars were smoothside cars, with some then getting stainless fluting applied over.  The method resulted in a lot of rust and corrosion issues, because water could become trapped between the steel and stainless steel layers.  Prewar cars were particularly prone to this, but it was also found even after postwar improvements in construction.  As with skirting, once the fluting was removed for maintenance or repair, it was often just left off.  A few roads like Santa Fe, which relied on fluted stainless cars almost as a trademark, had rigorous maintenance schedules for their cars in order to maintain their structural integrity along with the "looks".

As CBQ Fan said, Budd's construction method was different and more durable.  Budd invented and patented "shotwelding" which allowed them to efficiently weld their stainless panels directly to the structural framework of the car, and weld seams- creating a more watertight car, and being all stainless, was much less prone to rust or corrosion.

^This.  The C&O, as another example, removed the fluting from a lot of its passenger cars for this reason.

DFF

[Sokramiketes]

Couple other notes on the correct statements made here.

The Budd cars are also correctly referred to as "corrugated" as the structure of the sheets was integral to the car bodies.  With almost the entire cars being stainless, the lifespans have been amazing.  Just look to the VIA cars still in prime service.  Budd private cars are also a hot commodity as the sills/underframe structure have been granted almost infinite lifespan for Amtrak inspections.

The P-S cars were still Cor-Ten weathering steel bodies, with decorative "fluted" panels applied.  Which meant water could get behind the panels, and water is the enemy of Cor-Ten.  Cor-Ten needs to breathe, and form the protective oxide.  Rotting sides lead to the removal of the fluted panels. 

[cjm413]

^This.  The C&O, as another example, removed the fluting from a lot of its passenger cars for this reason.

DFF

Also RI