A long-distance train is a design load before it is a timetable entry. Rolling-stock planners fix the length of a consist years before the first passenger boards, and that single number then decides platform edges, passing loops and the radius of every curve the train must take. In the Bernese Oberland that number is now 400 m. As Plattform J reported on 8 October, citing BLS, today’s curves into Interlaken are too tight for trains of up to 400 metres. According to BLS, the line between Interlaken West and Interlaken Ost has been closed since Monday 19 October for eight weeks to change that.

The cascade starts further down my lakeside track. On the Thunersee line, BLS is stretching the Leissigen passing loop from 360 m to 940 m, a CHF 73 million step, so 400 m InterCitys can run Zurich to Interlaken. At Interlaken itself, about 1.7 km of track and switches between the western Aare bridge and Interlaken Ost are being replaced. Curves are widened, some tracks shift by more than a metre, and entry speed rises from 30 to 50 km/h. The closure runs in planned phases: west bridge to Marchgraben until 7 November, Marchgraben to Interlaken Ost until 27 November, then power and signalling through mid-December. A second closure, Spiez to Interlaken Ost, follows from 30 March to mid-May 2027.

Two riveted trusses, fifty more years

Then the river. BLS built the upper and lower Aare bridges in 1915, according to the federal tender record: skewed riveted truss bridges over two spans, both listed monuments. BLS states on its project page that the bridges are 110 years old, and that weak steel elements are being reinforced or replaced so they can carry trains for another 50 years. The reinforcement goes on the underside only. From the Aare promenade, nothing changes. The upper bridge also gets a new central pier, and the lower one gets renewed rust and scour protection.

Somewhere in those rivet lines is the work of a 1915 crew whose names nobody recorded. Their joints have outlasted every timetable the line has run. The steelworkers now under the deck are adding a second century to that labour. Walo Bertschinger AG Bern holds the main construction contract at CHF 13.59 million, according to the published award record.

←TODAY: About CHF 40 million total: track CHF 25.5 m, upper bridge CHF 8.8 m, lower bridge CHF 5.5 m.
→3012: A listed truss still running trains in its third century, because each generation strengthened it from below.
Fulcrum: The train got longer, so the geometry moved. The heritage stayed because the strengthening hid underneath.

The river is treated as a client too. Crews work partly from a floating platform and behind sheet-pile walls. BLS reports that the old corrosion protection contains lead, chromium and zinc. It comes off inside an airtight, filtered enclosure, the new coating is pollutant-free, and an independent environmental supervisor checks compliance. The same package nets the rock at the foot of the Harder and adds a 90 m long, 2 m high retaining wall.

Here is the trade-off. Keeping the listed silhouette means every new load path must fit inside geometry fixed in 1915, which leaves fewer options for the steel. PAZ has covered this Swiss habit before in What a bridge inspection can actually hear. Under SIA 269, old structures are recalculated against real site data rather than replaced. Interlaken shows that discipline at work.

Atelier: If your office designs near a station or a line, the rolling stock is a client you never meet. It sets clearances, platform lengths and the curve your façade sits beside. Monday move: add one line to your project’s design basis that names the governing vehicle (length, axle load, speed), with its source and date.

Hack: Size a curve from the speed you want and watch how fast the radius grows. Lateral acceleration rises with the square of speed, so going from 30 to 50 km/h almost triples the radius you need at the same comfort limit. The value of a_q below is illustrative, not BLS’s design figure.

a_q = 0.65  # m/s², illustrative lateral limit, no cant
for v_kmh in (30, 50):
    v = v_kmh / 3.6
    print(v_kmh, "km/h -> R_min ~", round(v**2 / a_q), "m")

The output is about 107 m and about 297 m. That jump is why tracks move by a metre in a dense town. Put the same relation into a Grasshopper slider in Rhino and drag it: the geometry follows the vehicle.

From where I run, in the late 2070s, the bridges that lasted were the ones someone kept recalculating, not the ones someone admired. Find the oldest structure your project leans on and ask who checks it next, and against which load.