I will be arriving at Frankfurt airport on 25th may with my wife and two children ,one 7+ and the other 9+ ,at 6.30a.m.I plan to travel to Interlaken and Luzern and return from there to Koln germany on 31st may. would you advice me two buy two country rail pass or just swiss pass and travel from Frankfurt to Basel bying the ticket at the airport.What is cheaper and convinient.
Let's see. You plan to fly into Frankfurt, but you want to go to Interlaken, which is much closer to Zürich, which has an airport. It would be much more convenient to fly into Zürich.
Assuming you are under some kind of constraints like using frequent flyer miles, or Air India only flies to Frankfurt, then I guess you have to take the train from Frankfurt.
So far you have only indentified 3 days of travel, Frankfurt to Interlaken, Interlaken to Luzern, and Luzern to Köln. A Germany-Swiss Saver Pass (5 days min) costs $366/pers, $1100 (about €800) total because the children pay half.
You can get a Europe-Spezial Schweiz fare from Lucern to Köln for as low as €78 for all four of you online from German Rail by purchasing in advance. These fares are date and train specific, limitedly refundable, and require an advance purchase of at least 3 days from the German Rail website (already in English). The tickets prices are tiered, and the price goes up as the lower prices sell out. For May 31, the lowest price I see still available is €118 for adults and 2 children (free under 15 if noted on the ticket) on the 9:54 train from Luzern. After that, later trains appear to have €158 fares available.
I see Europa-Spezial fares as low as €138 still avilable from FRA to Interlaken. However, due to the possibility of a late flight, using non-refundable, train specific fares on the day of arrival is problematic. Could you spend the night in the Frankfurt area and go to Interlaken the next day?
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How big a time buffer would you have to leave to assure making the required train? The ICE/IC connection to Interlaken takes 5 hours. If you would need a four hour buffer, that's nine hours. In less than 10 hours you could go from FRA to Interlaken using regional trains and a purchase-at-the-time Baden-Württemberg-Ticket for just €28 to Basel plus local fares from FRA to Mannheim-Waldhof (€27,20). (You would still need Bahn tickets from Basel Bad (the last station in Germany) to Interlaken, for €70,80 for all 4).
OTOH, full fare from FRA to Interlaken by ICE/IC is just €215, so you could use full fare to Interlaken, Europa-Spezial to Köln for €118, and still be about €450 less than using a pass. That €450 should more than cover your trip from Interlaken to Luzern plus whatever other travel you might want in Switzerland.
You don't need any passes.
Discount and Länder fares work very well in Germany, especially for a family.
I don't like the 2-country passes, which are only available in 1st class and often don't include children for free. On the other hand, a Swiss-only pass can work very well for a family, as the children are free with a family card. The parents would get Saver prices (15% discount) and you can use 2d class which is fine in Switzerland.
Since they have several days in Switzerland they will probably be using other trains or lifts, or boats in addition to the trip to Interlaken.
Two 3-day Flex passes for the parents would be 2 x 212 CHF, or 424 CHF total. This would cover from the border to Interlaken, from there to Luzern, and from Luzern back to the border. Any other travel they did within Switzerland (including cablecars and boats) would be 50% off. And if they visit any museums, that would be free.