SEPTA doesn't have enough engineers to run all of its trains all of the time.
On Saturday, eight Regional Rail trains were canceled because of crew shortages, and engineers say the problem is chronic and may get worse.
In 2011, federal work rules were toughened, reducing the number of hours passenger-train crews can work in a week or month and exacerbating SEPTA's long-standing staffing woes.
Because of a shortage of qualified workers, the complex nature of rush-hour scheduling, and SEPTA's desire to limit costs for employee benefits, all engineers and conductors work overtime every week and are paid accordingly.
Top-earning engineers and conductors make more than $120,000 a year, including overtime. The top rate for a SEPTA engineer is $30.10 per hour, and the top rate for a SEPTA conductor is $26.75 per hour.
SEPTA has 194 locomotive engineers, and 396 conductors and assistant conductors.
The agency needs 213 engineers to be fully staffed, said spokeswoman Jerria Williams. Five conductors who have completed training to be engineers will be on the job this week, she said, helping to ease the shortage, and 16 additional trainees are in SEPTA's yearlong training program to become engineers.
About 66 percent of the transportation authority's engineers are not available for weekend work because they already have worked all the hours that federal rules allow, she said.
Typically, crews cannot work more than 12 consecutive hours and six consecutive days, according to Federal Railroad Administration rules.
"We had 12 engineers who called out on Saturday," Williams said, referring to last weekend's shortage. "Having that many call out is high. . . . It's more like six or eight on average."
"People are working till they're too beat up to work," said Tom Dorricott, an engineer who is also a union representative for the Brotherhood of Locomotive Engineers and Trainmen, citing schedules that require some engineers to work 70 or 75 hours a week. "SEPTA just hasn't been hiring enough people."
Engineers and conductors have been working without new labor contracts since 2009. Both Dorricott and Williams said the labor impasse has not prompted crews to stay off the job.
Under federal law, railroad workers may not strike until negotiations and mediation have failed.
The big paychecks and long hours of railroad workers are tied to the unusual nature of their jobs. And the costs are boosted by a long history of employee benefits that are more lucrative - and more expensive - for railroad workers than for most other workers.
SEPTA has calculated that it's cheaper to have fewer rail crews working longer hours than more crews working shorter shifts.
Part of the equation is that railroaders' benefits are expensive. Employers and employees covered by the Railroad Retirement Act pay higher retirement taxes than those covered by Social Security, and the benefits are better.
The average age annuity being paid to career railroad employees at the end of 2009 was $2,228 a month, compared with $1,434 being paid to Social Security-covered retirees, according to government figures.
Rail employers pay 12.1 percent in "tier II" retirement taxes to finance benefits that exceed Social Security levels. But that tax applies only on earnings up to $81,900, so employers such as SEPTA avoid that tax on much of the money paid to high-earning engineers and conductors.
That's an extra incentive to pay overtime rather than to hire additional workers.
SEPTA is trying, however, to increase its number of engineers, Williams said.
"We are always trying to encourage conductors to apply. But it's a tough training program," she said - involving the intricacies of operating trains on every SEPTA route - "and not all of them graduate."