Ground conditions in two cuttings proved harder than the surveys had suggested, which stalled excavation for several months. Steel and signalling prices have also risen sharply since the contract was signed. On current projections the electrification works will overrun their budget and finish more than a year after the original completion date. However, the contractor has added a second tunnelling crew to recover some of the lost time. Because the line must stay open to commuters, the heaviest work can proceed only during weekend closures, and progress has consequently been slow.
Which statement expresses the central idea of the passage, rather than a supporting detail?