When was the first grade separated junction?

[tbtc]

My trivially minor question is when was the first grade separated junction? (i.e. intended to avoid conflicting moments over the flat)

Such things wouldn't have been needed in the early days, with so few trains, and possibly a lot harder to engineer, so in my mind I'd guess the early 1900s?

(For avoidance of doubt, I'm not talking about lines that 'happen' to cross over/under like Tamworth, but junctions specifically designed to avoid trains in opposite directions getting in each others way)

 

[norbitonflyer]

My trivially minor question is when was the first grade separated junction? (i.e. intended to avoid conflicting movents over the flat)

Such things wouldn't have been needed in the early days, with so few trains, and possibly a lot harder to engineer, so I'm my mind I'd guess the early 1900s?

Much earlier. 1844, at Norwood Junction.

It carried the experimental Atmospheric Railway track which ran from Forest Hill to Croydon (now West Croydon, the original terminus of the London & Croydon Railway). As this track was built on the east side of the existing tracks, it was necessary to cross the tracks towards (East) Croydon and Brighton - which had opened in 1841 - at Norwood Junction (then known as the Jolly Sailor) and, as the atmospheric system was not compatible with conventional points and crossings, a flyover was built.
Because of technical problems, the atmospheric trains only started operating in early 1846 - it was extended to New Cross (now NX Gate) a year later, but closed shortly afterwards.

As far as I am aware there is no trace now of the viaduct - the area has of course been remodelled numerous times, most notably the addition of the two routes from Victoria, via Crystal Palace and via Norbury, and quadrupling of the original line via Forest Hill. It was apparently made of wood, so may not have been strong enough to carry steam locomotives.