Regular SMUs (pictured top right), also known as SMU Mk Is, are recognised by the "SM" prefix on the carriage numbers (inside beside the doors) and their plain, flat, yellow ends with the numbers 201 to 212 on the bottom right. They were introduced from 1994.

The next-generation of SMUs (pictured bottom right), known as either SMU Mk IIs or SMU 220s, are recognised by the "SM" prefix on the carriage numbers (inside beside the doors) and their flat yellow ends with decorative blue patterning, with the numbers 221 to 250 on the bottom right. They were introduced from 1999.

For full information see the SMU fleet page.