Replacement priorities

[Baldy's Boss]

The bigger a company gets the more vehicle replacement is needed.When formats change there's no autoreplace shortcut (I've been maglev-izing,having to replace whatever trains use tracks and station regardless of age).So what replacements to do first becomes an issue.

Do you favor oldest first,or choose the most lucrative lines first,or those in most need of help that you don't want to just fold?

Examples from attached are welcome.