What are the typical speeds used on fibre links between COs in North
American and elsewhere ?
So, with a single pair of fiber, you can easily purchase a system (including amps) to do 40x10G.
How far you it reaches all depends on the equipment you use.
I realise that there are special cases, such as Shaw using WDM to reach
400gigabits/sec in a link in Western Canada. But what are the most
common speeds ?
Most common increment these days is OTU2 or ~10G client side. You can get 100G solutions from vendors as well.
Is WDM in common use to multiplex multiple 10gbps, 40gbps links ? Or is
that used mostly for intercity trunks and intra-city trunks tend to not
bother with WDM because of ample supply of fibre ?
Much of this depends on the existing services on a path, what's in-use, etc.
Do telcos typically quickly upgrade fibre strands to newer speeds, or
are there still many strands handling only 100mbps or 1gbps between COs
because telcos never got around to upgrading legacy equipment/services
(such as ATM etc) ?
I'm sure there are plenty of legacy services out there. I've seen pricing of ethernet-over-copper services for 5-10megs that are the same as existing T1 pricing. Carriers like to move away from the T1/DS1/DS3 business as it's lower cost and typically unregulated.
Also in terms of costing WDM, would one typically have a card that has
the multiple colours in the card and outputs a single multi-colour
signal or would one typically have separate cards for each colour and
then use a WDM "filter" to combine/separate colours onto the single strand ?
This depends on many things. I've seen people doing all variants of this, and they work with varying degrees depending on the equipment involved. You also will see that within datacenters or buildings where you may hit 1km, 2km or even 10km limits on a campus.
There should be plenty of vendors that will be happy to talk to you about active/passive cwdm/dwdm systems.