I've worked with RTLinux in automotive. It is only used in R&D and testing. It's much worse than INTime (windows rtos) or L4, which are usually used. I've even patched g++ to work on RTLinux eliminating all the dynamic, unreliable stuff.

People are using Linux when they need HW and driver support, e.g. gigabit Ethernet, firewire and such. RTOS vendors charging shitload of money on those drivers. I trust the Linux drivers more than the RTLinux scheduler or libc. But well, recently networking went to hell, so even there they start fucking up.