US chain bringing up regulations on public transport? (7)

'us chain' is the definition.
(US retail chain)

'up regulations on public transport?' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'regulations' becomes 'law' (regulations are laws).
'on' means one lot of letters go next to another.
'public transport?' becomes 'tram' (type of public transport).
'law' after 'tram' is 'tramlaw'.
'tramlaw' backwards is 'WALMART'.

'bringing' acts as a link.