Railway covered with cubes of frozen carbon dioxide (3,3)
I believe the answer is:
dry ice
'carbon dioxide' is the definition.
(thesaurus)
'railway covered with cubes' is the wordplay.
'railway' becomes 'ry' (abbreviation).
'covered with' means one lot of letters goes inside another.
'cubes' becomes 'dice' (I have seen 'Small cubes ' mean 'dice' so perhaps 'cubes' could also mean 'dice').
'ry' placed into 'dice' is 'DRY ICE'.
'of frozen' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for dry ice that I've seen before include "Solid carbon dioxide (used for a stage effect)" , "White mist on stage" , "Theatrical mist" , "may be used for effect on stage" , "Carbon dioxide snow" .)