Building Much Needed Rail Grade Separations Act of 202 1 This bill establishes a grade separation investment program to award competitive grants to certain entities for projects that separate railway-highway grade crossings, improve safety by reducing the number of deaths and injuries at rail-highway crossings, increase the efficiency and reliability of the movement of people and freight over rail-highway crossings, increase the capacity of the freight and passenger rail system, reduce the amount of noise from trains, and improve air quality and reduce greenhouse gas emissions from less idling at rail-highway crossings. Among other requirements, the bill (1) requires grant awards for large projects (projects with total costs of greater than $100 million) to be at least $50 million, and for any other projects not less than $2.5 million; and (2) directs the Department of Transportation to consider a number of factors in awarding grants, including accident history at the rail-highway crossing over the last 10 years, the annual average daily motor vehicle, cyclist, and pedestrian traffic at the crossing, the likelihood of a collision based on the geometry of the crossing, and the number of critical facilities near the crossing.