Something I was kind of curious about are the single pin fluorescent fixtures. Bi pin fluorescent tubes are most common from what I see, it is used just about everywhere. Seems like its easier to replace single pin tubes than bi pin since you don't need twist them into place. Which applications are the most common use for single pin? On transit vehicles (buses, trains, etc.), are those usually single or bi pin fixtures? I don't have any bi pin fixtures or tubes so I am very new to this. Thanks.

Are you refers to the old european single pin fluorescent lamps that designed to be started directly from the 220-240V mains with a switchstart/preheat magnetic ballast without a starter, that have the same pins as the bi-pin lamps, or the Slimlines, which designed to be operated from the high OCV of the american instant start autotransformer ballasts, and have larger and shorter pins?
Anyway, single pin lamps, are best suited for street lighting and applications where the lamp operate continuously or started one time per day, as these lamp have no source of cathodes heating beside the discharge itself.

The tubes i see in busses and trains here are the normal bi pin lamps as used everywhere. But the busses and trains here are pretty new (busses mostly from 2000s, in the 90s it was all incandescent, and trains from early 90s)

And there used to be spring loaded sockets for bi pin tubes, in them you replace the lamp in the same way as you would a single pin lamp

I was referring to the T12 and T8 single pin fluorescent tubes. They don't seem very common as all the fluorescent tubes on the store shelf are all bi pin type. So I assume they are used more for commercial applications.

You're thinking of slimlines, which are instant-start lamps, with (T12 ones anyway) usually magnetic ballasts

I had some Sylvania VHO tubes a while back, they were single pin

Id never heard a single pin VHO if it exist that new to me. They generally have RDC endcaps wich is two
Pin lamp recessced  double contact as those still two
Pins. Wich may  look like single pin lamp
Here a good example of a RDC cap for HOs and
1500 mA lamps.

http://www.lighting-gallery.net/gallery/displayimage.php?album=2193&pos=47&pid=73312

Id modded this lamp