What distinguishes M-series equipment from other passenger cars?

The distinguishing feature of M-series equipment is that they belong to specific series, namely the M-3, M-7, or M-8. This classification highlights the design and operational characteristics of these particular passenger cars, which are built to cater to suburban and urban rail services. Each version within the series has specific enhancements and modifications suited for different operational demands, while still maintaining a standardized identification system.

This specificity in naming helps to categorize these passenger cars based on their design features, technology enhancements, and intended use, allowing for better maintenance, construction, and operational practices tailored to their performance requirements. Other types of passenger cars may not belong to such a defined series, leading to a varied range of features and capabilities.