Minimum population for metro?

[Somnolence 81]

This can actually apply to SC4 too, but I'm asking specifically for C:SL.

So, cities develop at much lesser populations in the game than in real life. At least from my experience.

The least populous city with metro I've seen is Genoa (wiki link here), with around 600k inhabitants. On the other hand, the cities I build rarely go over 100k, even if they're dense and highly developed. That being said, what's the adequate population to start building the metro network? Or does it, perhaps, depend on more factors?

[csSniper 1]

I build my metro in a 120u grid design and find that its much easier to do if I do it before I even layout the city streets. So I lay out the metro grid while I still have a 0 population. You should do it however it's easiest for you.