The plug looks similar to a poppet valve. The problem was traced to a malfunction of the vernier LOX poppet valve. These inefficiencies drove the widespread experimentation in poppet valve gears for locomotives. The T1 was also the first production series locomotive designed to use the poppet valve. The 68 Special was an inline poppet valve design. In 1931 No. 2935 was rebuilt with rotary cam poppet valve gear. The valve core is a poppet valve assisted by a spring. Francis Stevens invented the Stevens valve gear, a double beat poppet valve, in 1839. The poppet valve gear gave the engine extremely free-running characteristics. In the early days of engine building, the poppet valve was a major problem.