 Sections with a driving trailer are called head sections; those with an intermediate trailer are called intermediate sections.The sections cannot operate independently; the minimum train length is 2 head sections, and the maximum is 6 sections (2 head sections and 4 intermediate sections), with a train 5 sections long (2 head and 3 intermediate) considered standard.Coupling more than 6 sections is unsafe due to overloading of motor-generators which power the engineer's controls.