Indoor becoming an outdoor?

[Mississippilessly]

We got our lovely car from a rescue 2 years ago. She had been a stray. Due to a mild cough they requested that she was rehomed as an indoor cat. We first lived in a flat and she was fine. However last year we moved to a house and she is getting more and more interested in outside. It's becoming harder to keep her in and a bit of a pain. I feel so guilty keeping her in. However, she does still have the cough. We also live on a main road and the house backs on to a railway line.
DH is against her going outside because of the safety aspect. Any thoughts please? I feel bad as we work long hours but I will be on maternity leave soon and will be off for 14 months. I can't see a harness working and due to the number of trees in the garden I can't cat proof it.
Thoughts?

If the back garden allows it, maybe look into building a Catio? They can get a bit of outdoor time at least that way, while being contained safely.

If it backs onto a railway line, I think your DH is right. Can you cat proof the garden?

As there's three reasons for keeping her in (main road, railway line, health issues) a catio that is around your back door seems the safest option, and then strictly never allow her out at the front of the house.

My parents adopted and indoor cat. He'd lived in a flat, very quickly after getting him it became clear that he wanted out. So they had to let him as he got aggressive. (He's very soppy otherwise)

Thanks for the replies. DH won't entertain a catio. I've tried.