‘M’ for Metra System. A single color shared across all lines, with services delineated by number. Numbers increase radially from north to west to south, and downtown terminals concentrate a range of numbers.
For example, at Union Station you have access to
M4North Central Service
,
M5Milwaukee District North
,
M6Milwaukee District West
,
M7BNSF
,
M8Heritage Corridor
, and
M9SouthWest Service
.
At Ogilvie you would have access to
M1Union Pacific North
,
M2Union Pacific Northwest
, and
M3Union Pacific West
.