Function to Simulate Realistic Waveform
It varies with the situation.  In the real world, there will be some transmission line effects, mainly filtering out higher frequencies but also some reflections.  I found a circuit lab schematic/simulation someone named signality created with a lumped-element model of a transmission line, and I've slightly modified it.







simulate this circuit &ndash; Schematic created using CircuitLab

The simulation is set up to plot with R2 (the load termination) equal to 30 ohms, 50 ohms (nominal), and 70 ohms.  

You can change this test bench to simulate your own source and export csv data to use in your simulation. It probably won't match your situation exactly (particularly the frequency scaling), but take a look at the shapes below.

I know you only asked about triangle waves, but square waves and sawtooth waves are also interesting.  They are worse than triangle waves because of the higher-frequency components involved in sharp transitions.

Square wave:



Square wave (faster):



Sawtooth:



Sawtooth (faster):


Triangle:

You can see how there are minor "humps" in the trace due to the reflections.

Triangle (faster):