Now, a question: Can I use the Sleep spell twice (...) ?
Yes, you can.
I like that plan. I believe we all have some longe range weaponry at hand, I would use my cantrip ot not lose an axe, so once I've "enlightened" the rail we would all have a good shot at the goblin. I would think that in reality we'd all shoot at the same time more or less, so no idea how this is handeled in D&D context but I belive a surprise attack should include the whole party. Let's just hope our dice rolls are more fortunate this time and that there is nobody else around.
The sleep speel would be the most elegant and sneaky approach by far though. Since you can use it twice I think I would go that route. Would redeem us a little after the disaster that was our last engagement.
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just a clarification, the goblin appears to be on the bridge
Yes, he would have to run to one end and try cutting it loose twice. No idea how thick the ropes are but I would assume that would be two turns.