In the railway industry, making a time management schedule for freight and passenger train plays a vital role in optimal use of railway resources such as tracks, fleet and human resources. In the last few years, there has been a growing interest in using non-periodic mathematical models for scheduling of train movements between researches. In the main mathematical model of train scheduling, which is used in a variety of single-objective and multi-objective models, there are some constraints that prevent the collision of trains with each other. The present study tries to take a closer look at the models and reduce these constraints. To this end, in this article an improved non-periodic train scheduling model is suggested by presenting a new definition of headway time. Our claims and results of the improved and the main models are evaluated using two hypothetical and one real examples. The results illustrate that the new proposed model, which has less constraints and complexity, works more effectively than the main model and is more suitable for solving real train scheduling problems.