Photo lifted from Ebay.

1) What was behind the oval windows? Restrooms?

2) From what we see here, can we guess the car manufacturers?

Speaking of the car on the right, I have seen that window before. It is used on Sacramento Northern car Bidwell in both toilet compartments. The car was rebuilt in the railroads shop, when these windows were added. I have also purchased one of these windows on e-bay. It was from a mainline railroad car in the east. I believe these windows were factory made and sold to railroads and car builders.

If you note the round piece in the middle, you will see where the top piece appears to be set in. From the inside a passenger can actually loosen a thumb screw and swing this half circle around, leaving the upper half of the circle open as a vent. These are very well made windows.

I have also seen these oval windows used in the corner of a car in the closet that holds the Baker heater. Noting the stack on the roof, that might be what is behind this window. I have no information as to the builder.

"2) From what we see here, can we guess the car manufacturers?"

Look like 80 foot Pullman "Palace Cars" from the early 1890's era...

... at least, to me.

The oval sash were typically used in "saloons" (toilet rooms). These sash were cast metal (brass? bronze?) with an inner and outer casting held together with screws, with the art glass held in between. They were available either with or without the pivoting vent window. Adams & Westlake was still showing them in the 1922 Car Builder's Dictionary, but they were long out of style by that time.

Of course, a carbuilder could use these wherever the customer wanted, but I don't think the one in the photo is in front of a stove; a Baker style heater would have the enclosure for the expansion tank above. Instead I see a small Globe vent, which was typical for saloons. The other vent looks to me like the vent used over the closet that held an acetylene generator for acetylene lighting. These were often located adjacent to a saloon bulkhead.

With certain notable exceptions, it is pretty hard to tell the builder of a wood car by the styling. Styles changed, and all the builders followed the trend. The triple sash under the arched art glass sash is somewhat unusual, but not indicative of any particular builder. The photo shows a car number; time to start researching the Rock Island roster to see who built number 1096.

Hicks built CA&E 309 interurban car also has an oval window in the bathroom.
The center section has the rotating vent that can open to let air in.
http://www.irm.org/gallery/CAE309/aau
From the Hicks Car Works site, similar car 319.
http://2.bp.blogspot.com/_YGfyS0_koOE/S ... 09+001.jpg
Maybe if Randy sees this he can post a better picture of the window.

Buzz is correct; the CA&E 300-series cars were all built with windows like this, though some were later rebuilt without them. IRM has two cars that still has these windows, 309 and 319; the best photos I was able to find of the window are of the 319:

http://www.irm.org/gallery/CAE319/aam
http://3.bp.blogspot.com/_YGfyS0_koOE/TDNKceyYqkI/AAAAAAAAFv0/UWSzt3_hms4/s1600/June+5,+2010+IRM+CA%26E.JPG

Note that in the second photo the semicircular vent pane has been partly opened.

Thanks for the replies everyone. Interesting reading.