<<"The direct route goes under a long tunnel. The Kandersteg route goes through a Pass a has much better views (and a short tunnel). Haven’t checked the timetable but maybe adds a bit over an hour depending on connections.">>
Actually, it goes "through" a long tunnel, to be precise. ;-)
Both get you to Visp, where you must change for Zermatt. Spiez is where the routes diverge. It takes an hour and 30 or 32 minutes to get from Zürich main station to Spiez. All trains go through Bern. Some require a change at Bern ( 6 minutes allowed) some do not require a change.
C heck the timetable which can tell you which platform to use when you leave Zürich, if you wish to be sure to get the train from Zürich which does not require a change at Bern, They leave at almost the same time from Zürich, so it can be confusing.
( A "type A's delight"). Plan ahead, ;-)
It is quite a scenic ride up and over the old route from Spiez onward, which does go through a short tunnel just past Kandersteg. Both the ascent and descent (in the Rhone Valley) have striking scenery, different because of the geography. If you choose it, sit on the right, especially once you are past Kandersteg. It takes 56 minutes longer to get to Visp.. Because the old route goes to Brig - very close to Visp- there is a change of trains (included in the 56 minutes) for the 6 minute ride to Visp, thence onward to Zermatt.
To represent in total time consumed, rather than the difference, after Spiez, - the new, fast tunnel, which has no interesting scenery inside the tunnel, takes 26 minutes to make the stretch to Visp from the last common point on both routes...Spiez. The old route takes 1 hour 21 minutes, including ( on one schedule I checked), 7 minutes to change trains at Brig and 6 minutes to get to Visp.
The next question - how much more time does it take to get all the way to Zermatt, once again, from Spiez as the last common point. This takes into account schedule matches at the various transfer stations, if any.
1 hour 38 minutes new fast route, either 2 hrs 39 minutes or 3 hours 2 minutes on some trains via scenic older route. Be careful about schedule matches.
Kids get bored with scenery pretty fast. At least one more mature forum visitor who took the upper route wrote afterward that the scenery in Zermatt was what they really came to see, so the slower route basically was not nice enough to compensate for the extra time. To each his or her own. I like the old route a lot,but have not tried it with kids in a hurry.