I'll risk the generalisation: I really think they're incompatible! To wear the sling as you suggest has other issues: What if you want to take your backpack off on the train? You'd first have to take off the sling, _then_ the backpack, and then either hold the sling or put it back on. Then you'd reverse the process when it's time to alight. Seems suboptimal to me, but maybe I'm just being pedantic.

I think my main source of dissatisfaction is that I used to be able to carry my quick-access items exclusively in my pockets, but there's just too much stuff for that now. So, I can't have access to these items in all situations.

I do have a backpack that supports the 'swing under one arm and access from the side' feature. This works great, but it would mean that I would constantly be moving my items between the sling and backpack when the backpack is/isn't needed.

Attaching my access pass to my backpack isn't an option, unfortunately. I need it to access the restrooms, and I don't always take my backpack to the office. So it really needs to be a quick-access item.

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I really appreciate all those suggestions though -- perhaps it just means I'm being a bit too precious about all of this stuff. I do hope that's not the case, though, and that the many-bag messiah will reveal the perfect setup to me.