Which option correctly describes a Train Describer Number (TDN) with four digits?

Which option correctly describes a Train Describer Number (TDN) with four digits?

• A. Train Describer Number (TDN)
• B. Train Describer Number (TDN) with four digits
• C. Train Describer Number (TDN) with five digits
• D. Track Describer Number (TDN) with four digits

Answer: (B)

The concept being tested is the exact description and format of a Train Describer Number used in signaling. The correct choice specifies that the Train Describer Number is four digits long, which matches how these identifiers are defined in practice: a four-digit numeric code that uniquely identifies a train for trackside communication and control systems. This precise four-digit length makes it unambiguous and easy for signals, dispatch, and onboard systems to process quickly. Why this is the best answer: it states both the term (Train Describer Number) and the required four-digit format, leaving no ambiguity about the length. The other options either omit the digit-length detail, propose an incorrect five-digit length, or use an incorrect term (Track Describer Number) that does not match standard nomenclature.

The concept being tested is the exact description and format of a Train Describer Number used in signaling. The correct choice specifies that the Train Describer Number is four digits long, which matches how these identifiers are defined in practice: a four-digit numeric code that uniquely identifies a train for trackside communication and control systems. This precise four-digit length makes it unambiguous and easy for signals, dispatch, and onboard systems to process quickly.

Why this is the best answer: it states both the term (Train Describer Number) and the required four-digit format, leaving no ambiguity about the length. The other options either omit the digit-length detail, propose an incorrect five-digit length, or use an incorrect term (Track Describer Number) that does not match standard nomenclature.