Pravin Thakre (29), who was in a live-in relationship with the victim Roshni, was arrested in Pune, police said.
"We arrested Pravin in the afternoon and he is being taken to Mumbai," said Suryakant Noukudkar, inspector, Pune Railway police.
According to police sources, Pravin and Roshni were staying together since 2002, and had a four-year-old child from the relationship. However Pravin fell in love with another girl, which caused spats between the two.
Roshni was killed on September 26. Her body, stuffed in trolley-bag, was found at CST on the morning of September 26.
The post-mortem report indicated that victim had been assaulted and strangled.
CCTV footage at CST showed two persons aged between 25-30 years -- one of them Pravin -- standing with the bag on the platform for a while after getting down from Pune-Mumbai Sinhgad Express, and then rushing away, leaving the bag behind.