What is the most difficult part of a running gear overhaul?

I have enjoyed the responses to my questions concerning overhauling a steam locomotive boiler. Now I have a few concerning the running gear.

What is the most time consuming part of overhauling a typical rod driven steam locomotive's running gear? Geared locomotive's running gear?

What is the most critical part of the running gear to keep maintained?

What part of the running gear receives the most wear during normal, whatever that might be, usage?

If money, materials and skilled labor are not an issue, how quickly could a rod locomotive's running gear be overhauled? A geared locomotive's running gear?

Thanks for answering these questions. Very interesting stuff.