On a recent Monday morning aboard a subway train in Manhattan, a man sat blasting music from a device, seeming to contravene rules requiring music to remain below a certain decibel level. Fellow passengers on the train tried to ignore it. It was harder to overlook his other violation: His pet was not in a carrier.

A tortoise balanced on his knee. Every so often it peered around the moving train car.

The host of rules governing conduct on the city’s subway system has evolved over time to weigh in on things like acceptable noise levels, the wearing of roller blades (no), and more.

Yet the prevalence of man-spreaders — men who splay their legs and hog seats — and riders who believe their backpack needs its own seat shows that subway scofflaws are as common as rats. But in this season of discontent, with failures of the subway’s antediluvian signal systems snarling commutes and stranding travelers, and with delays ballooning to over 70,000 a month, there is a growing sense that the rules are being tossed out the window more and more.

The explanation, in the view of many riders, is straightforward: Why respect rules when the subway fails repeatedly to respect their desire to get places in a reasonable amount of time?