{{binbou kusai} tanin to issho no} ressha nanka ni noru-n-jaa nakatta wa... 貧乏くさい他人と いっしょの列車なんかに 乗るんじゃあなかったわ・・・ I shouldn't have boarded a train {together with {[all these peasants]}}.
{binbou kusai} tanin 貧乏くさい他人 People [who] {smell of poverty}. (literally.)
jaa じゃあ In this case, same as ja じゃ, but she pronounces it longer.
shitei-seki tottatte koe ga tsutsu-nuke jaa nai 指定席とったって 声がつつぬけじゃあない Even though [they] call [it] a reserved seat, [you] can hear voices through the walls, can't you?
She's complaining there's nothing special about the seat, since she can hear the peasants talking anyway.
to-ttatte とったって to itta tte と言ったって to itta toshite-mo と言ったとしても to itte-mo と言っても Even if you call it. Even if you say it is. (contraction.)
tsutsu-nuke 筒抜け To be overheard.
konna-n-dattara inemuri dekinai kedo kuruma de kuru-n-datta wa こんなんだったら 居眠りできないけど 車でくるんだったわ If it's like this I wouldn't be able to sleep but I should have come by car.
Since janakatta can mean you shouldn't have done something, it makes sense that datta can mean you should have done something.