Polar coding is a low-complexity method for communication over noisy classical channels, which is capable of providing highly reliable data transmission. This paper proposes an enhanced polar codec scheme for mission critical applications in train-to-ground wireless communications. Firstly, we develop an enhanced polar coding scheme to support reliable data transmission under time-varying channel conditions. Especially, we design a channel reselection algorithm, where the optimum channels are used to convey mission critical applications. Secondly, we develop a system-level enhanced train-to-ground transmission protocol (eTGTP) for delivering the polar codes information and channel state information (CSI) between transmitter and receiver. We investigate the performance of polar codes in a linear cellular scenario, and the simulation results are also provided to obtain useful design insights.