In this paper, we consider an uplink grant-free access scheme for machine-type communications (MTC) in a railway scenario, inspired by the semi-persistent scheduling approach and motivated by railway operation environment monitoring. The scheme assumes a co-existence scenario, grouping critical MTC (cMTC) and massive MTC (mMTC) users and assigning them recurring uplink slots in a non-orthogonal multiple access fashion. Our focus is on assessing the age of information (AoI) performance of the users, which is a metric that combines timeliness and reliability, and as such is highly relevant for monitoring and control applications in modern railway transportation systems. By assuming a Rician fading channel characteristic of railway scenarios, we derive the successful reception probability and the average and peak AoI in reception scenarios that assume capture effect and successive interference cancellation (SIC). We show that the proposed scheme can achieve favorable performance approaching that of an ideal access method in which the users always have exclusively reserved resources whenever they want to transmit. We also investigate optimization of the periodicity of the resources scheduled for the proposed scheme and power allocation for the users' transmissions under the constraints on the average AoI gaps with respect to the ideal performance and provide illustrative results. Finally, we compare the performance of the proposed scheme to an AoI-aware solution that incorporates packet deadlines, showing that our scheme achieves lower average AoI and, at the same time, is capable of more effectively using resources in the uplink.