In practice
Doubles and triples are combinations of a tractor and two or three shorter trailers, usually 28 ft "pups". They're common in less-than-truckload (LTL) freight, where the extra trailer lets one driver move more freight per trip. A Class A CDL plus the T endorsement is the license that work asks for.
Source: 49 CFR 383.93(c); 383.141 · checked Oct 2026
The knowledge test covers coupling and uncoupling multiple trailers, inspecting them, and handling them on the road, including how rearward amplification makes the last trailer swing more. EXAMPLE: a driver adding T while studying for a Class A takes one more knowledge test at the DMV, then the T appears on the CDL once it's issued.
Source: 49 CFR 383.93, 383.153 · checked Oct 2026
Carriers that run doubles usually want the endorsement plus experience on single trailers first, because a set of doubles handles differently, especially when empty or in wind. Ask a recruiter how they train new doubles drivers before you count on a doubles job straight out of school.
See every endorsement side by side in CDL endorsements. Planning hazmat too? Try the hazmat endorsement practice test and the hazmat endorsement cost and timeline calculator.
Questions
Can I put the T endorsement on my permit?
No. Only P, S and N can go on a commercial learner's permit. You pass the doubles/triples knowledge test and the T is added to your CDL.
Are triples allowed everywhere?
No. Federal rules allow doubles on the national network, but triple trailers are allowed only in some states and on some routes. The endorsement lets you drive them; state law decides where.