the fastest engine available at the time
even my vacuum trains have 2 engines
This doesn't tell me which locomotive you are using. Is it a default vehicle ? Because the SH'40 and even the AsiaStar aren't that good at hauling freight. However, the X2001 and Lev1/2/3s are perfect for freight hauling.
You can also get into the Arctic / Tropical climate. They have much, much more powerful diesels there.
Note that generally if you use any NewGRF vehicle set (2cc TrainsInNML, DB Set XL, etc), there won't be the default vehicles.
I strongly reccomend you the two sets above. Be aware that if 2cc TrainsInNML works in all climates, DB Set XL only works in Temperate. I strongly reccomend the DB Set XL if you're looking for a set with a lot of vehicles but not too complex, because the vehicle list with 2cc Trains is so long that i'd give 100€ to anyone that has managed to use ALL vehicles.
well i use the default trains
and my question was general
so the engine would not be an isure
even if i use the weak asia star as you say or even the ginu a4
the question is the same
will be the same a 2 engine ginzu 7 tiles train
or 1 engine ginzu a4 3.5 tiles long
both configuration reach the maximum speed
so i was asking from the acceleration point
and also from the point of having
2n small trains versus having N longer ones
To say, the 3,5 tile train will accelerate faster. If you'd want the 7-tile to accelerate as fast as the 3,5 tile, you would need to put 3 locos.