I have a ticket for a conventional railway line. Can I take the Shinkansen instead (or vice versa)?

I had this question earlier so I did my research, and I thought some people might eventually ask the same question. The answer is: likely, if you're not doing crazy things to your ticket. The details are a bit tedious, but it concerns a basic principle of JR ticketing, "Shikansen and conventional lines are seen as equivalent" (in Japanese, shinzai d??ichishi ?????). Hereafter, we will simply call this the "equivalence rule".

Historically, Shinkansen lines were constructed to replace (and complement) the existing main lines (kansen) for long-distance travel, and that is how they got their name. Thus, Shinkansen lines and the conventional line running in parallel with them are deemed the same line by JNR (and the successor JR companies) for ticketing purposes. There are three Shinkansen systems with parallel conventional lines:

• the T??kaid??/San'y??/Ky?sh? Shinkansen: T??kaid?? Main Line, San'y?? Main Line, and Kagoshima Main Line
• the T??hoku Shinkansen: T??hoku Main Line
• the J??etsu Shinkansen: T??hoku Main Line (T??ky??~??miya), Takasaki Line (??miya~Takasaki), J??etsu Line (Takasaki~Nagaoka), and Shin'etsu Main Line (Nagaoka~Niigata)

(The Hokkaid?? Shinkansen does not have a parallel conventional line, because there are no longer regularly scheduled passenger services running on the Kaiky?? Line.)

If part of a ticket's route is one of those "parallel Shinkansen lines", the Shinkansen can be taken instead.

Example: I have a base fare ticket (j??shaken ???) from ??saka (city zone) to Okayama. When I bought the ticket, I intended to use the Shinkansen, but now I'd like to use the express trains on the T??kaid?? Main Line and San'y?? Main Line instead. I can do this, because the T??kaid?? Main Line and San'y?? Main Line runs in parallel with the San'y?? Shinkansen. OTOH, if I have a conventional line ticket (which would have something like ?????????? printed on it) and I didn't do strange things when I bought the ticket, I can still use the Shinkansen, provided that I buy a limited express ticket (tokky?ken ???).

Exception 1 (special intervals): Shinkansen lines and existing conventional lines do not parallel each other completely, and JR consider Shinkansen lines and conventional lines equivalent only if they actually run in parallel. There are 16 intervals in which Shikansen lines and their "parallel" conventional lines do not actually run in parallel, and if the starting point, ending point, or a connection point of your trip is located within one of those intervals, the equivalence rule stops applying to you at the first point where the two lines diverge from each other.

The 16 special intervals are (the endpoints of the intervals are not included):

• on the T??kaid??, San'y?? and Ky?sh? Shinkansen (& the parallel conventional lines), between: Shinagawa~Odawara, Mishima~Shizuoka, Nagoya~Maibara, Shin-??saka~Nishi-Akashi, Fukuyama~Mihara, Mihara~Hiroshima, Hiroshima~Tokuyama, Hakata~Kurume, Chikugo-Funagoya~Kumamoto;

• on the T??hoku Shinkansen (& the T??hoku Main Line), between: Fukushima~Sendai, Sendai~Ichinoseki, Ichinoseki~Kitakami, Kitakami~Morioka;

• on the J??etsu Shinkansen (& the parallel conventional lines), between: Kumagaya~Takasaki, Takasaki~Echigo-Yuzawa, Nagaoka~Niigata.

Example: I have a ticket from Nagoya to Yokohama through the T??kaid?? Main Line. After I buy the ticket, I found out this is a stupid idea because the conventional line will take me forever. However, I cannot switch to the Shinkansen without changing my route, because my destination, Yokohama, is between Shinagawa and Odawara (and indeed, the T??kaid?? Shinkansen does not stop at Yokohama). I can change my route at a ticket window, so that I ride the Shinkansen all the way to Odawara and change to the conventional lines only there. In this case, this is a mere technicality, because the operational distance (and thus, the base fare) is the same.

Example: I want to go from Okayama to Gifu. To do this, I want to pick up a connection at Nagoya. Now, without the special intervals rule, the segment from Nagoya to Gifu would be backtracking and won't be allowed on one single ticket.

However, since Gifu is in the Nagoya~Maibara exceptional interval, we can do this on a single ticket: the operational distance is simply Okayama to Nagoya (366.9 km) plus Nagoya to Gifu (30.3 km), i.e. 397.2 km, and the fare is 6,480 yen.

Note that if we change to conventional lines in Maibara instead of Nagoya, the operational distance would be Okayama to Maibara plus Maibara to Gifu (which will be equal to Okayama to Gifu on the T??kaid?? Main Line). The distance is 333.4 km and the fare is 5,620 yen.

Exception 2 (Shinkansen within large city zones): There are five "large city" zones (???????) designated by the JR companies. For travel within one of those zones, passengers can (1) travel using any route regardless of the designated route and pay the same fare, subject to the no backtracking and no loops rules as usual (in practice, this means one always pays for the shortest route), and (2) no stopovers are allowed. Nevertheless, with only two exceptions, large city zones do not contain the Shinkansen routes otherwise completely within those zones. The two sole exceptions are the intervals between Maibara and Shin-??saka, and between Nishi-Akashi and Aioi (both are within the ??saka large city zone).

However, for travel within large city zones, the equivalence rule still applies. In fact, both rules apply, making the situation very complicated. But, in short:

• if a ticket for a conventional line is taken, you may not use the Shinkansen unless the Shinkansen route is considered equivalent to the route designated on your ticket, or part thereof. If you use the Shinkansen, the normal rules that apply to travel within a large city zone still hold.

• even if only part of a ticket's designate route contains a Shinkansen route, the trip will be deemed to be outside of a large city zone. The normal rules of route selection and stopovers apply. The Shinkansen equivalence rule still applies as usual, so one may potentially use this exception to circumvent some of the large city zones rules, without actually intending to use the Shinkansen. (Not personally tested, though.)

Example: I have a ticket from Odawara to ??miya, and my base fare ticket has the following designated route (the last line, in Japanese, is what would be printed on the ticket):

Odawara - (via T??kaid?? Main Line) - Chigasaki - (via Sagami Line & Yokohama Line: Sagami Line) - Hachi??ji - (via Hachik?? Line & Kawagoe Line: Hachik?? Line) - Kawagoe - (via Kawagoe Line) - ??miya
???????????????????????????

(Note: this is extremely unpractical and perhaps very costly.)

Since my travel is within the T?kyo large city zone, I may use any route that does not involve backtracking. So, I might as well use the more practical route: take T??kaid?? Main Line train to Yokohama, and then take the Keihin-T??hoku Line train to ??miya (via the T??kaid?? Main Line and the T??hoku Main Line).

Now, I might want to save time and take the Shinkansen. I can take the T??kaid?? Shinkansen to T??ky??, then transfer to the T??hoku Shinkansen. But with the ticket above, we cannot do so, because the T??kaid?? and T??hoku Shinkansens are not part of the Tokyo large city zone.

However, if we have the following itinerary, we can use the Shinkansen instead:

Odawara - (via T??kaid?? Main Line: T??kaid?? Line) - T??ky?? - (via T??hoku Main Line: Keihin-T??hoku Line) - ??miya
????????????

Note that we can't pick up a connection at Yokohama, but rather only at T??ky?? or Shinagawa: Yokohama is within the Shinagawa~Odawara interval, and the Shinkansen equivalence rule does not hold. The ticket is still only valid for one day, and no stopovers are allowed.

Example: Suppose I want to get to Odawara from Matsumoto using the Super Azusa Limited Express, but I also want to spend a night in Tokyo to get some rest. Since both Odawara and Matsumoto are in the Tokyo large city zone, we normally can't do this on a single ticket. However, we can abuse the equivalence rule to get a stopover, as Odawara and Matsumoto are over 200 km apart by rail.

To get the stopover, we buy a base fare ticket with this itinerary without any intention to actually use the Shinkansen:

Odawara - (via Shinkansen) - T??ky?? - (via Ch??? Main Line) - Shinjuku - (via Ch??? Main Line & Shinonoi Line: Super Azusa) - Matsumoto
????????????????

Since this trip consists of a Shinkansen segment, it's not contained within the Tokyo large city zone. As this itinerary is more than 200 km, stopovers are allowed, and the ticket is valid for 3 days.

Since the T??kaid?? Shinkansen and T??kaid?? Main Line run in parallel between T??ky?? and Odawara, we may (ab)use the Shinkansen equivalence rule and use the T??kaid?? Main Line instead. Then, we can take a stopover in T??ky?? and stay for a night. The next day, we buy a limited express ticket, get to Shinjuku, and catch the Super Azusa there. But note that we will no longer be able to use any route within the Tokyo large city zone: we must stick to our itinerary instead.

Finally, a personally tested example:

Example (personal): I have a base fare ticket from the Osaka city zone to Matsuyama with a connection in Okayama. The itinerary states that I will the San'y?? Main Line, Honshi-Bisan Line and the Yosan Line, in succession. Moreover, I want to get stopovers in Himeji and Okayama to see the Himeji Castle and K?rakuen Garden. However, for the Osaka (city zone) to Okayama part, I decided to take the Shinkansen later.

So, I go to Shin-??saka and buy two Shinkansen limited express tickets there: one for Shin-??saka~Himeji, and one for Himeji~Okayama (note that limited express tickets do not allow stopovers). I take the Shinkansen to Himeji, leave the station, tour the castle, and come back to take the Shinkansen to Okayama. I get another stopover there, tour the garden, and finally come back to catch the Shiokaze Limited Express train to Matsuyama. Since none of my connection points are in a special interval, I do not have to worry about fares. Easy peasy.

References:

1. JR East website: ???????
2. JR West website: ?????????????????
3. Japanese Wikipedia: ??????? (JR)