This is the set of instances use in the PACE 2018 competition, of optimal Steiner Tree computation. The instances are grouped into three tracks of 200 instances each, except for the third track which is only 199 instances. Each instance is an undirected graph.
Track 1 is the "exact with low number of terminals" track, Track 2 is the "exact with low treewidth track", and Track 3 is the heuristic track. The exact tracks were intended to test solvers that need to provide a provably optimal solution; the heuristic track was intended for solvers that produce good but possibly suboptimal solutions. Details of the tracks and problem setup are on the PACE problem description page. The data format is specified in Appendix A of that page.
Graphs have sizes (number of vertices) ranging up to several thousand for exact tracks (1 and 2), and up to tens of thousands for the heuristic track. The exact tracks are typically very sparse; some of the heuristic instances are dense.
The official data download is on the PACE GitHub.