I may have found my perfect commuting bag!
A few months ago (after several months of commuting with my two-bikes-plus-train solution), the thought occurred to me, "Wouldn't it be great if I could pull this pannier along through train stations (between bikes) instead of carrying it?"
I seriously began going through my (extensive!) collection of wheeled holdalls, business cases and suitcases, looking for a suitable candidate that could be reinforced and to which I could fit pannier hooks (Klick-fix for preference).
And then I spotted this on ebay:
This case holds as much as my 20L Ortlieb City Roller.
With the added important feature of an outside pockets for lights, which I have to fit and remove from two different bikes each morning and then again each evening.
Typical contents for the day:
The rack clips are situated so that the bike sits high enough to reduce risk of heel strike. This is especially important since the bottom of the bag is square, not tapered like many panniers are to address this issue. I have not noticed any effect on handling, since I tend to put heavier items at the bottom anyway.
There are three clips, all spring-loaded. Their positions are fixed horizontally as they are attached to rods inside the lining at the rear of the bag. That is to say, the clips can't be adjusted to the left or to the right to accommodate the location of any bars on your rack. Fortunately, the positions work on both the Tubus Logo rack on the Cross Check and the Blackburn rack on the Viking Bromley.
The only 'weakness' in what otherwise suits my commuting needs to a tee is the lack of total waterproofness. I do watch the weather forecasts quite carefully, so if heavy and/or prolonged rain looks possible, I use one of my Ortlieb panniers. Otherwise, this bag is very nearly perfect. I may consider using a small "drybag" inside it for items I really don't want to risk getting damp.