Eric Brosius wrote:
Depends how much it costs, how much time you save, and how likely it is that you'll re-use the track.
A rule of thumb I use (not specific to Martian Rails) is that a turn is worth about $3 million.
When I first started playing, I recalled you (or someone) stating this number as an average for turn. I kept that in mind when building track. However, I later used a spreadsheet to track everything during a few games, and found that even a losing score averages over $5 per turn. The winning scores are around $7 per turn, or higher.
I'd say the rule of thumb is if the build will save you one turn per $5 spent, then it is acceptable, more so if you have time to use that track again! This rule can break down in the final few turns of the game, when you just need to break $250 to trigger the win before someone else makes their "final" deliveries.