unn els ▁were ▁intended ▁to ▁be ▁built ▁with ▁a ▁diameter ▁of ▁, ▁large ▁enough ▁to ▁accommod ate ▁both ▁a ▁main ▁line ▁train ▁type , ▁and ▁the ▁O H LE ▁w ires . ▁ ▁At ▁the ▁same ▁time , ▁British ▁Rail ▁was ▁in ▁the ▁midst ▁of ▁testing ▁a ▁new ▁sub urban ▁train ▁type ▁in ▁the ▁Southern ▁Region . ▁These ▁prototype ▁units , ▁eventually ▁designated ▁as ▁Class ▁ 4 4 5 ▁and ▁ 4 4 6 ▁under ▁the ▁Total ▁Oper ations ▁Process ing ▁System ▁( T OP S ), ▁were ▁different ▁from ▁any ▁previous ▁rolling ▁stock : ▁they ▁were ▁constructed ▁of ▁al umin ium ▁rather ▁than ▁steel , ▁were ▁an ▁all ▁in ▁one ▁design ▁rather ▁than ▁a ▁coach ▁body ▁on ▁top ▁of ▁a ▁separate ▁under frame , ▁and ▁had ▁sl iding ▁doors . ▁Although ▁the ▁Class ▁ 4 4 5 ▁and ▁ 4 4 6 ▁were ▁only ▁prototype ▁units , ▁the ▁information ▁gar ner ed ▁from ▁their ▁use , ▁both ▁in ▁initial ▁testing ▁and ▁passenger ▁service ▁in ▁the ▁Southern ▁Region , ▁and ▁later ▁in ▁department al ▁use ▁as ▁test bed ▁units , ▁led ▁to ▁the ▁production ▁of ▁a ▁new ▁series ▁of ▁sub urban ▁EM Us ▁that ▁came ▁into ▁use ▁in ▁the ▁mid ▁to ▁late ▁ 1 9 7 0 s . ▁ ▁In ▁ 1 9 7 5 , ▁SE L NE C ▁( by ▁then ▁renamed ▁as ▁the ▁Gre ater ▁Manchester ▁Pass enger ▁Transport ▁Executive ) ▁published ▁a ▁new ▁Pic c - V ic ▁public ity ▁bro ch ure ▁that ▁went ▁into ▁greater ▁detail