Said every other train's interior is covered with streaks (8)
I believe the answer is:
striated
'covered with streaks' is the definition.
I can't judge whether this definition defines the answer.
'said every other train's interior' is the wordplay.
'said' becomes 'stated' ('state' can be a synonym of 'say').
'every other' means one should take alternating letters.
'interior' means one lot of letters goes inside another.
The alternating letters of 'train' are 'ri'.
'stated' enclosing 'ri' is 'STRIATED'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for striated that I've seen before include "Groovy" , "Striped" , "Marked with ridges in tardiest way" , "marked as a zebra" , "Marked with linear shapes in tardiest way" .)