You’d have to do it using nodes for curves, but that applies whether you use SetPrimaryPartCFrame with lerp, CFrame every part or Tween it. You can’t make it follow a curved path without having nodes, so you tween from one node to the next.
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You don’t have to use lots of nodes to make a curve. As I recommended before, you can use a function to create a curve. (ex: y = x^2)
Bezier Curves provide a relatively simple way to do this.
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I have made it move. Turning just looks funky. I am probably not setting my nodes how they are supposed to be. I put a node with the same orientation as the track wherever the railroad turns.
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Do you adjust for the change in rotations? Lerping with CFrames will automatically adjust rotation along with the position.
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Depends on how you produce the CFrame input. Tween will do the rotation to if you provide it with a CFrame that has rotation. I use it for cars and trains.
It may be how you’ve attached the rest of the train to the tracking parts. Did you use alignposition and alignorientation or a weld or something?
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I used a lot of welds. Each part in each car is welded to the PrimaryPart of that car, and then I Lerp the PrimaryPart’s CFrame to each node.
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I did not. I’ll have to do that. Should it adjust right after or before the track turns?
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The lerp should do the rotation if you use the full CFrame of the node as the goal, and not just the position part of it. Mind sharing the code with your inputs to the lerp function and setting the primary part to the result? Might help isolate the issue.
Also if your nodes are physical parts, make them visible to double check the orientation. If they are virtual then perhaps share the code that generated them to double check orientation is correctly set there too.
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Its best to adjust it along the way.
local part = Instance.new("Part")
part.Anchored = true
part.CFrame = CFrame.new()
part.Parent = workspace
local CF1 = CFrame.new()
local CF2 = CFrame.new(0,10,0)*CFrame.Angles(0,math.pi/2,0)
for i=0,1,0.01 do
part.CFrame = CF1:Lerp(CF2,i)
wait()
end
Lerping CFrames instead of Vector3s means you don’t have to worry about updating the position and the rotation separately.
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Edited code so people don’t steal. 
function GetTime(Distance, Speed)
local Time = Distance / Speed
return Time
end
function movemodel(model,start,End,AddBy)
for i = 0, 1, 0.01 do
if not model.PrimaryPart then break end
model:SetPrimaryPartCFrame(start:Lerp(End,i))
RunService.Heartbeat:Wait()
end
end
local Distance = (script.Parent.PrimaryPart.Position - nextnode.Position).magnitude
local Time = GetTime(Distance, Speed) -- Calculate the time
local AddBy = 1/Time
movemodel(script.Parent,start,End,AddBy)
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That looks like it should work. I’d double check your nodes are the correct orientation by making them visible for the moment, and possibly output the angles of start, End and the lerp result for debugging - if they all show the correct values then something else is going on.
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Thank you so much for the replies @Halalaluyafail3 @BanTech @marinatedsushi. Finally got it to curve the right way. 
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How did you manage to follow the railroad the right way? I tested your edited code, but it won’t work. Mind sharing the not edited code with me? I can do better with your code. I am a developer, I wish to join you in your game xCrim
You can adjust the orientation as exactly as the railroad turns. CFrame contains position and rotation
This is an old post. I ended up making a much better version after I left the XCrim Dev Team. But I haven’t been able to get on studio in a week or so.
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How did you manage to make the train go straight and follow curves? Mind sharing your undedited code because i tried it and before i can reply to you i was not a new member yet. Can you share your non edited code with me?
Been a while. I’ll post this file here because I’ve been getting Discord dms about it.
In this file it’s a minecart instead of a train but it’s still the same concept. You can try converting it into a train.
Trainstuff.rbxl (126.6 KB)
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Is there any way I could add multiple train cars / carriages that are coupled to the main train in that file?
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Easy. This file contains both the Train and minecart.
PiggysTrainSystem.rbxl (128.8 KB)
Not gonna make a tutorial on this. People need to learn to either make your own stuff or learn by looking at how the code is used.
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I find a way to make something different.
Automatic Train System - Resources / Community Resources - Developer Forum | Roblox