Well there weren't many ramen shops open on the day after New Year's (1月 1日), so I took a ride up to Akihabara and then walked further to Okachimachi. The shop Tareso (it's actually in Ueno according to the map + address) was the only one that I could easily find near JR Okachimachi or Ueno-Hirokoji station (Ginza line) that was open. Because it was very cold I had the regular chashu ramen/chashumen. It was served very hot and the broth had the right shoyu flavor but it tasted a bit weak, not a very strong taste. The noodles were thicker than spaghetti, like fat spaghetti, and the regular size was < 200g, I would get the large size if possible. The chashu was very good and generously portioned, it was the long slice style which was slightly richer/fattier on the top half and darker and leaner on the bottom. A boiled egg and menma completed the picture. The shop is laid out with a very long counter stretching all the way to the back of the shop and for a big gaijin like me it was difficult to get all the way into the back without bumping into anyone or knocking their coats off the wall. However the seats were reasonably widely-spaced though.
MenWorld blog entry (more pictures + hours)
Diddlefinger Map (English labels)