A thousand queuing to catch a fast train here? (4,4)
I believe the answer is:
main line
'a fast train here?' is the definition.
(a fast train might run on a main line)
'a thousand queuing to catch' is the wordplay.
'thousand' becomes 'm' (Roman numeral for a thousand).
'queuing' becomes 'in line' (standing in line).
'to catch' indicates putting letters inside.
'm'+'inline'='minline'
'a' put within 'minline' is 'MAIN LINE'.
(Other definitions for main line that I've seen before include "Inject drug - principal rail connection (8;4,4)" , "Principal rouote of a transportation system" , "Take 9 intravenously" , "Chief part of a rail network" , "Way to travel" .)