I see a myopic lack of cost/benefit analysis in this discussion.
Let's say you have a city with production 20 per turn and it is 10 tiles from your capital.
To connect it with a railroad (without modifiers) would cost maintenance 10*2=20 per turn.
Post-railroad production would be 20*1.5=30 per turn.
For a 300 hammers-cost (just to make for round numbers), it now takes you 10 turns to build, during which you are paying 200 maintenance on the railroad. Without the railroad it would have taken you 5 extra turns to build. That's 100 hammers at the old production speed.
So you're spending 200 gold to save 100 hammers. Is there some reason that gold is only valued at half a hammer? Note this is not even including the extra maintenance from keeping workers around to lay rails.
If you're planning on building something like the Utopia Project which can't be purchased, in a city which is not your capital, then _maybe_ building railroads right before beginning said production is worthwhile. Otherwise, given the OODA-loop benefit of having the immediate purchase option...why would you make this trade?
Where could my example/analysis be wrong. Well, what if your average "production city" is not 10 turns away from your capital but only 5. Now RR maintenance is 10 per turn, so you're trading 100 gold for 100 hammers. That sounds more reasonable (though I strongly suggest that the immediate purchase option has a HUGE tactical benefit which is not being quantified here.)
So how often do you have a major production city which is 5 tiles or fewer from your capital? That's pretty much impossible in my games, simply because the capital is always one of the larger cities and is almost certainly working quite a few of the tiles which a nearby city would need to be a production powerhouse.
What's the other benefit from a railroad - 1/3 movement cost or thereabouts. Compare with a road which is 1/2 movement cost for half the maintenance. The tactical benefit of this increased movement depends on your specific situation, but...how long are you sitting around paying maintenance on railroads before using that movement benefit in a battle inside your own borders? Couldn't you have insta-purchased a unit (or more) where needed for what you paid in maintenance on the railroads?
Conclusion: MAYBE in specific cases where you plan WELL in advance to make the best use of the railroad's hammers-for-gold trade, railroads are worthwhile. It seems likely that this celestial alignment will be rare.
Final note: I am not at all a proponent of realism in Civ (it's a game, not a simulation) but it's interesting to note that this treatment of railroads is completely bass-ackwards. Historically, railroads cost a lot to build but result in improved commerce AND production once completed. Like roads, with judicious placement the trade route benefits of railroads should (realistically) at least pay for the maintenance cost.