Really hard to do without knowing the layout of the building or the area as Astro said, but here goes:
1. Can you hang out nearby and observe this potential entrance, or do something to it so anyone going in/out would make it noticeable to you? It's possible that IS the entrance. A friend and I went to a place the only way in was crawling under a gate which was on the corner of two relatively busy roads. The only thing that could be done is to wait for a break in traffic, go for it, and hope/pray you weren't seen by anyone who cares.
2. Look on google maps, maybe there's a way to approach from the rear. Unless the building is flush with the drop off to the tracks, I'd be willing to bet there's a way to walk around it. Hell, maybe if you're lucky there will be a manhole around back and you can just use the drains for your approach.
3. Can't say much here, really depends on what this business is, their hours, etc. They may be using the abandoned part as storage, who knows. Can you get into this business and possibly gain access to the abandoned part from the inside? Just a thought.
4. Like Granuaile said, go super early if hanging out in the ghetto during the day doesn't sound like fun. Pick a time to get out and be out by then. Knowing what the road/sidewalk traffic is like can help you pick a good time before things get too busy. I don't really suggest running unless you have to, just walk with a sense of purpose and GTFO of that area. Running is likely to draw more attention than just walking like you belong, IMO.