At first, I must explain that "SANDO" is shorter version of a word sandwich. Then, Japanese do not say "I ate sando for lunch" but say "I ate ham sando for lunch" instead of "I ate ham sandwich for lunch". Tuna sandwich will be tuna sando in Japan.
Then, a type of sandwich you hardly see in Europe is Katsu-sando. Katsu means deep fried meat with breadcrumbs = Japanese schnitzel. Tonkatsu means pork schnitzel due to "ton" means pork.
Tonkatsu is a popular dish in Japan and katsu-sando too. It is told that katsu-sando was invented in 1935 by a Tonkatsu restaurant ISEN in Tokyo. As like sandwich was named after by an 18th century English aristocrat who loved to eat the meat tucked between two pieces of bread during playing cards, Japan had a custom to eat food by hands therefore creation of various types of sandwiches were made.
Anyway, katsu-sando is a king of Japanese sandwich, the price is the most expensive one you can find in restaurants and convenience stores. However, you must try this when you are in Japan and it is ideal meal to bring on board of long distance journey.