In order to keep down the construction costs, the location where the bridge could be the shortest was chosen. This meant the S bend shown had to be negotiated by the railway, and as it happened the highway of the time followed a similar deviation. Here is another map which shows a lot more of the road and railway.
Leeston Road is the main road and in order to get across the Selwyn River, by the time the line closed, there was a bridge right alongside the rail bridge, with the two roads at each end, which are called Old Bridge Road North and Old Bridge Road South. The real question is why there were two bridges at all. When the railway was built, it was an era when combined bridges were common, and such a bridge would have not been inconvenient to the travelling public because there were not that many trains on a branch line. So why was there not a combined bridge (as far as we know)?
Probably because there was a ford for road traffic to begin with. The current highway bridge is at Chamberlains Ford – the access road to the old bridge now goes down into the riverbed to get to the well known picnic spot and camping area. So the highway bridge possibly did not come until later, and the rail bridge was probably unsuitable for conversion as such. At any rate, all the maps I have for the branch show two separate bridges at the location, right next to each other. Today there is no trace of the highway bridge, but the south abutment of the rail bridge is still visible, made of concrete. The north end is not easily accessible as it is on private property.
At the south end, the road (rail in the grass to the left) used to go straight ahead onto respective bridges. The road now goes down into the Chamberlains Ford camping and recreational area.
The rail bridge abutment at the south side.
Road approaching the crossing from the north side – the railway was immediately to its right.
Although in that era most main roads were notoriously twisty – traffic speeds and densities being far lower – this sort of bend in a railway is still relatively uncommon to see on the flat Canterbury plains, as it was not necessary due to terrain, which often leads to a lot of curvature. I would guess the two curves were fairly sharp and would have slowed trains, but in fact rail speeds on a branch would not have been very high – probably 40 km/h maximum. Another common trick with bridges in the early days of railway construction in NZ was to make them lower than the rest of a line, which means a short sharp grade at each end descending down. Even today the Waipara River bridge on the MNL is like this.