More than 700 MTA subway cars are considered to have outlived their “useful lifecycles” as the agency waits for long-delayed replacements.

The MTA’s subway cars are designed to be in operation for 40 years, but some 735 cars have surpassed that benchmark, according to an update on train car deliveries presented at an agency committee meeting on Monday.

Many of those trains are currently in service on the A, C, R, J and Z lines. Their old age shows in more ways than just bumpier rides.

The two oldest models, known as the R32 and R42, also break down much more frequently than their younger counterparts. Consider that the R32 fleet broke down once every 32,327 miles over the past twelve months of available data. The fleet-wide average during that time frame was 113,179 miles.

These cars’ replacements (the R179) were originally set to serve the riding public in 2017, but manufacturing delays at Bombardier, the company under contract to produce the trains, have pushed that date back and inflated the original $600 million price tag to $740 million.

The MTA now estimates that it will receive the full order of 300 new cars in July 2018. The agency received two five-car test units this fall.

The goal is to get the trains rolling before the L train shutdown in 2019. The retirement date for the oldest cars is still unknown.