Northwest China is an important juncture of the Silk Road Economic Belt. Considering the heterogeneity of railway traffic flow in different cities, with the main cities in the northwestern region as the research unit, and the important high-speed line openings in 2011, 2015, and 2019 as time points, using the accessibility of the train frequency measurement method and the standard gravitational model, and adopting the frequency-weighted average travel time of trains and the total economic ties and economic indices of the contact strength, the accessibility of railways and the urban spatial interaction are analyzed. The results indicate the following. (1) Railway accessibility and the strength of the economic connection between cities have significantly improved overall, especially along the high-speed railway, which is greatly affected by the train frequency. (2) Railway accessibility in the entire region has a significant gradient from east to west and gradually decreases in the northwest direction. In some provinces (autonomous regions), a circular structure is centered on the provincial capital city and sub-centered on the railway hub city. (3) The spatial interaction in the entire region has changed from the traditional “polar core” pattern to a “point-axis” pattern with the hub cities as nodes and the main railways, such as the Longhai, Baoji-Lanzhou, and Lanzhou-Xinjiang railways, as longitudinal axes, forming a “network” pattern. Within the provinces (autonomous regions), the “regional central city-peripheral city-marginal city” economic connection total gradient structure is obvious. It is suggested that railway construction be further strengthened; in particular, the coverage of the railway network in marginal areas should be expanded and the east-west gradient development should be coordinated. Cities along non-high-speed railway lines should strengthen their transportation connections with cities along the railway lines and actively integrate into the high-speed railway network.