Build Up the Suburbs Act or the BUS Act This bill modifies programs that support public transportation projects, with a particular focus on fixed-guideway transportation systems (e.g., light rail and bus rapid transit). Specifically, the bill expands eligibility for certain grants that support the construction and expansion of such systems. In particular, it increases the cap on federal assistance for Small Starts projects (funded through Capital Investment Grants) from $100 million to $360 million and the total estimated net capital cost from $300 million to $600 million. It also includes express lane transit systems within the scope of fixed guideways. Further, the Department of Transportation must give priority in selecting Nationally Significant Multimodal Freight and Highway Projects to those that include public transportation components.