locomotive and car axles

[nedthomas]

PRR electric locos GG1 used a "quill drive" which was a solid axle inside a hollow axle. This allows two motors per axle to drive the wheels. Do a "Google" on "quill drive" for more info.

[beaulieu]

Solid.  In transit applications, sometimes they are hollow.

 

Also hollow on high-speed trainsets like the TGV. 

[oltmannd]

Solid.  In transit applications, sometimes they are hollow.

[wisandsouthernkid]

are the axles on locomotives and the cars hollow or solid steel? i would imagine they would be solid but are they?