Costain had ranked top of the spending league table for the previous two years helped by its major station contracts.
But after peaking at £541m in 2016/17 spending dropped 22% to £422m.
The extent of Network Rail’s reliance on Carillion was also revealed with the firm’s construction and powerlines businesses being paid £378m over the 17/18 financial year.
A transfer of Carillion contracts to Amey in the latter part of the period helped to lift Amey Rail through the rankings.
Buckingham Group Contracting, Skanska Construction and Volker Rail featured in the top suppliers league for the first time this year, while Atkins and Geoffrey Osborne crashed out of the top 20.
The top 20 suppliers shared out £3.59bn, down 2% on the previous year, which accounted for half of Network Rail’s total supplier spend.