Something I've heard from Deltic drivers was that the climb up Holloway was a bit of a battle with the ammeter every time, mostly because of their high gearing. The concern was possible overheating or a flashover, and it probably didn't help that the old King's Cross layout was restricted to 8mph, so it was impossible to accelerate until going uphill in the tunnels (usually damp and greasy). In general, they couldn't open up to full power until New Southgate, and most logs show them topping out at 50-60. The Cravens DMUs on suburban services could beat that, and a Derby hydraulic unit could blow their doors off. Once the gradient became easier, drivers would give it the beans and rapidly build up to around 70-80.
Electrification used to produce 'races' between Deltics and 313s between Hatfield and Welwyn (which is also uphill). When a 47 ran VICE Deltic, the Flying Soctsman would sometimes be overtaken by a tiny 313 that had just started. Electrics can produce an almost-continuous tractive effort at low speed, so they tend to be very snappy off the mark.
Once over the top at Woolmer Green, it was possible to get a Deltic above 100 without over-stressing the motors, so drivers would just put them in top notch and leave them there. Curiously, 31s often showed a similar speed profile, although they were restricted to 90 and made their first stop at Stevenage, so in general they were just getting going when they had to hit the brakes.
