Era 1 Engines
The general consensus is to use the Swallow, fully upgraded, until the Falcon becomes available.
I agree in part, as in I agree to use the Swallow in such a manner but I wouldn't fully upgrade it, I would not bother to research acceleration.
The Swallow has the following upgrades: Large steam dome (+1 Tractive force), Inlet pipe (+2 Acceleration), Steam header (+10km/h Speed) and Safety valve (+20% Reliability).
Reliability must be taken during the tutorial. Tractive force is the biggest upgrade and should be researched right after as it adds a solid 33% to the cargo load, and profitability; also, must be taken to research the Raven. Speed upgrade doesn't add much, but at this point you've got 2 out of 3 and you have an engine that is not beaten until you research the Donkey. Acceleration doesn't make much of a difference, don't buy it and save the points.
When I first researched it I paid attention to the engines while they were being upgraded and the difference in revenues was around 2%. Now, according to my finances I have done 8 millions before swapping to Falcons, of those 8 millions some have been earned by the Career Engine, let's say that out of 8 engines took 1/8 of the revenue, 1 million, even though it was definitely more as, compared to the Swallow was a bigger earner, also, in the early days while upgrading the station was taking a higher part, anyway, that leaves us with 7 millions.
2% of 7 millions is 140k. Minus the 21k required to installed the update we have 119k. I've earned more than that in Competitions alone...
In my opinion, 4 research points are worth more than that and should not be wasted, better save them and get to the Falcon a few hours earlier. The reason being that the upgrade is so useless is that the Swallow has a very low top speed, which is lowered even more as conditions worsen, meaning that the train spend very little time accelerating.
Now, I said the engine is unbeaten until you unlock the Donkey, let's see why.
Raven with the upgrade to research Rhino has the same stats as the Swallow with half the reliability and a third of the acceleration. Due to poor acceleration you are not going to make much use of the higher 10km/h of top speed and the low reliability will make it expensive to repair. If you were to research the Whale the expenses wouldn't be as bad, but a total lack of acceleration would still mean not benefiting from that top speed.
Rhino isn't bad, if fully upgraded, but standard with only the required research to move on only has a 10% more on reliability and 5 points in acceleration, and since we agreed above that acceleration is almost useless when the top speed is low, this means that the Rhino has the same performances of the Swallow, and to make it perform better we would need to waste research points which can be used instead to move towards the Falcon.
Donkey is the first engine that even without upgrades is better than the Swallow, marginally, it takes a good lead after the 2 required upgrades to research the Falcon, thing is, at that point it costs 143k per piece, times 7 engines that's a million, for 7 engines that will be replaced a day and a half later as soon as the Falcon is researched. I can't be arsed to do the math now, but even at a glance I'd say replacing the engines costs you more than sticking to the Swallows and swapping for Falcons.
I have not fully researched the Mole, however, it would appear to me that is better than the Falcon only where waiting times are high, whereas for the waiting times that I have experienced so far in Era 1 and 2, around the 30/35 seconds in worst case scenario, more often less than 20, Falcon leads.
I suppose the Mole could have a post in competition scenarios (US East vs West) or if you have to haul goods from industries you don't control, but that's pretty much about it...
Falcon Upgrades: Flux compensator (+1 Tractive force), Heat controller (+3 Acceleration), Coupling wheel set (+20km/h Speed) and a Control system (+1 Acceleration).